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Short Description
A minimal docker baseimage to ease creation of X graphical application containers
Full Description

A minimal docker baseimage to ease creation of X graphical application containers

This is a docker baseimage that can be used to create containers able to run any
X application on a headless server very easily. The application's GUI is
accessed through a modern web browser (no installation or configuration needed
on client side) or via any VNC client.

Images

Different docker images are available:

Base distribution Tag Size
Alpine 3.5 alpine-3.5
Alpine 3.6 alpine-3.6
Alpine 3.5 alpine-3.5-glibc
Alpine 3.6 alpine-3.6-glibc
Debian 8 debian-8
Ubuntu 16.04 LTS ubuntu-16.04

Due to its size, the Alpine image is recommended. However, it may be harder
to integrate your application (especially third party ones without source code),
because:

  1. Packages repository may not be as complete as Ubuntu/Debian.
  2. Third party applications may not support Alpine.
  3. The Alpine distribution uses the musl C standard library instead of
    GNU C library (glibc).

Note that using the Alpine image with glibc integrated (alpine-3.5-glibc
tag) may ease integration of applications.

The next choice is to use the Debian image. It provides a great compatibility
and its size is smaller than the Ubuntu one. Finally, if for any reason you
prefer an Ubuntu image, one based on the stable 16.04 LTS version is
provided.

Content

Here are the main components of the baseimage:

  • S6-overlay, a process supervisor for containers.
  • x11vnc, a X11 VNC server.
  • xvfb, a X virtual framebuffer display server.
  • openbox, a windows manager.
  • noVNC, a HTML5 VNC client.
  • NGINX, a high-performance HTTP server.
  • stunnel, a proxy encrypting arbitrary TCP connections with SSL/TLS.
  • Useful tools to ease container building.
  • Environment to better support dockerized applications.

Versioning

Images are versioned. Version number is in the form MAJOR.MINOR.PATCH, where
an increment of the:

  • MAJOR version indicates that a backwards-incompatible change has been done.
  • MINOR version indicates that functionality has been added in a backwards-compatible manner.
  • PATCH version indicates that a bug fix has been done in a backwards-compatible manner.

Tags

For each distribution-specific image, multiple tags are available:

Tag Description
distro-vX.Y.Z Exact version of the image.
distro-vX.Y Latest version of a specific minor version of the image.
distro-vX Latest version of a specific major version of the image.
distro Latest version of the image.

Getting started

The Dockerfile for your application can be very simple, as only three things
are required:

  • Instructions to install the application.
  • A script that starts the application (stored at /startapp.sh in
    container).
  • The name of the application.

Here is an example of a docker file that would be used to run the xterm
terminal.

In Dockerfile:

# Pull base image.
FROM jlesage/baseimage-gui:alpine-3.6

# Install xterm.
RUN add-pkg xterm

# Copy the start script.
COPY startapp.sh /startapp.sh

# Set the name of the application.
ENV APP_NAME="Xterm"

In startapp.sh:

#!/bin/sh
exec /usr/bin/xterm

Then, build your docker image:

docker build -t docker-xterm .

And run it:

docker run --rm -p 5800:5800 -p 5900:5900 docker-xterm

You should be able to access the xterm GUI by opening in a web browser:

http://[HOST IP ADDR]:5800

Environment Variables

Some environment variables can be set to customize the behavior of the container
and its application. The following list give more details about them.

Environment variables can be set directly in your Dockerfile via the ENV
instruction or dynamically by adding one or more arguments -e "<VAR>=<VALUE>"
to the docker run command.

Variable Description Default
APP_NAME Name of the application. DockerApp
USER_ID ID of the user the application runs as. See User/Group IDs to better understand when this should be set. 1000
GROUP_ID ID of the group the application runs as. See User/Group IDs to better understand when this should be set. 1000
SUP_GROUP_IDS Comma-separated list of supplementary group IDs of the application. (unset)
UMASK Mask that controls how file permissions are set for newly created files. The value of the mask is in octal notation. By default, this variable is not set and the default umask of 022 is used, meaning that newly created files are readable by everyone, but only writable by the owner. See the following online umask calculator: http://wintelguy.com/umask-calc.pl (unset)
TZ TimeZone of the container. Timezone can also be set by mapping /etc/localtime between the host and the container. Etc/UTC
KEEP_APP_RUNNING When set to 1, the application will be automatically restarted if it crashes or if user quits it. 0
APP_NICENESS Priority at which the application should run. A niceness value of -20 is the highest priority and 19 is the lowest priority. By default, niceness is not set, meaning that the default niceness of 0 is used. NOTE: A negative niceness (priority increase) requires additional permissions. In this case, the container should be run with the docker option --cap-add=SYS_NICE. (unset)
TAKE_CONFIG_OWNERSHIP When set to 1, owner and group of /config (including all its files and subfolders) are automatically set during container startup to USER_ID and GROUP_ID respectively. 1
CLEAN_TMP_DIR When set to 1, all files in the /tmp directory are delete during the container startup. 1
DISPLAY_WIDTH Width (in pixels) of the application's window. 1280
DISPLAY_HEIGHT Height (in pixels) of the application's window. 768
SECURE_CONNECTION When set to 1, an encrypted connection is used to access the application's GUI (either via web browser or VNC client). See the Security section for more details. 0
VNC_PASSWORD Password needed to connect to the application's GUI. See the VNC Password section for more details. (unset)
X11VNC_EXTRA_OPTS Extra options to pass to the x11vnc server running in the Docker container. WARNING: For advanced users. Do not use unless you know what you are doing. (unset)

Config Directory

Inside the container, the application's configuration should be stored in the
/config directory.

This directory is also used to store the VNC password. See the
VNC Pasword section for more details.

NOTE: By default, during the container startup, the user which runs the
application (i.e. user defined by USER_ID) will claim ownership of the
entire content of this directory. This behavior can be changed via the
TAKE_CONFIG_OWNERSHIP environment variable. See the
Environment Variables section for more details.

Ports

Here is the list of ports used by container. They can be mapped to the host
via the -p <HOST_PORT>:<CONTAINER_PORT> parameter. The port number inside the
container cannot be changed, but you are free to use any port on the host side.

Port Mapping to host Description
5800 Mandatory Port used to access the application's GUI via the web interface.
5900 Optional Port used to access the application's GUI via the VNC protocol. Optional if no VNC client is used.

User/Group IDs

When using data volumes (-v flags), permissions issues can occur between the
host and the container. For example, the user within the container may not
exists on the host. This could prevent the host from properly accessing files
and folders on the shared volume.

To avoid any problem, you can specify the user the application should run as.

This is done by passing the user ID and group ID to the container via the
USER_ID and GROUP_ID environment variables.

To find the right IDs to use, issue the following command on the host, with the
user owning the data volume on the host:

id <username>

Which gives an output like this one:

uid=1000(myuser) gid=1000(myuser) groups=1000(myuser),4(adm),24(cdrom),27(sudo),46(plugdev),113(lpadmin)

The value of uid (user ID) and gid (group ID) are the ones that you should
be given the container.

Locales

The default locale of the container is set to POSIX. If this cause issues
with your application, the proper locale can be set via your Dockerfile, by adding these two lines:

ENV LANG=en_US.UTF-8
RUN locale-gen en_CA.UTF-8

NOTE: Locales are not supported by musl C standard library on Alpine.
See:

Accessing the GUI

Assuming that container's ports are mapped to the same host's ports, the
graphical interface of the application can be accessed via:

  • A web browser:

    http://<HOST IP ADDR>:5800
    
  • Any VNC client:

    <HOST IP ADDR>:5900
    

Security

By default, access to the application's GUI is done over an unencrypted
connection (HTTP or VNC).

Secure connection can be enabled via the SECURE_CONNECTION environment
variable. See the Environment Variables section for
more details on how to set an environment variable.

When enabled, application's GUI is performed over an HTTPs connection when
accessed with a browser. All HTTP accesses are automatically redirected to
HTTPs.

When using a VNC client, the VNC connection is performed over SSL. Note that
few VNC clients support this method. SSVNC is one of them.

Certificates

Here are the certificate files needed by the container. By default, when they
are missing, self-signed certificates are generated and used. All files have
PEM encoded, x509 certificates.

Container Path Purpose Content
/config/certs/vnc-server.pem VNC connection encryption. VNC server's private key and certificate, bundled with any root and intermediate certificates.
/config/certs/web-privkey.pem HTTPs connection encryption. Web server's private key.
/config/certs/web-fullchain.pem HTTPs connection encryption. Web server's certificate, bundled with any root and intermediate certificates.

NOTE: To prevent any certificate validity warnings/errors from the browser
or VNC client, make sure to supply your own valid certificates.

NOTE: Certificate files are monitored and relevant daemons are automatically
restarted when changes are detected.

VNC Password

To restrict access to your application, a password can be specified. This can
be done via two methods:

  • By using the VNC_PASSWORD environment variable.
  • By creating a .vncpass_clear file at the root of the /config volume.
    This file should contains the password in clear-text. During the container
    startup, content of the file is obfuscated and moved to .vncpass.

The level of security provided by the VNC password depends on two things:

  • The type of communication channel (encrypted/unencrypted).
  • How secure access to the host is.

When using a VNC password, it is highly desirable to enable the secure
connection to prevent sending the password in clear over an unencrypted channel.

Access to the host by unexpected users with sufficient privileges can be
dangerous as they can retrieve the password with the following methods:

  • By looking at the VNC_PASSWORD environment variable value via the
    docker inspect command. By defaut, the docker command can be run only
    by the root user. However, it is possible to configure the system to allow
    the docker command to be run by any users part of a specific group.
  • By decrypting the /config/.vncpass file. This requires the user to have
    the appropriate permission to read the file: it has to be root or be the
    user defined by the USER_ID environment variable. Also, to be able to
    retrieve the correct decryption key, one needs to know that the content of
    the file was generated by x11vnc.

DH Parameters

Diffie-Hellman (DH) parameters define how the DH key-exchange is performed.
More details about this algorithm can be found on the OpenSSL Wiki.

DH Parameters are saved into the PEM encoded file located inside the container
at /config/certs/dhparam.pem. By default, when this file is missing, 2048
bits DH parameters are automatically generated. Note that this one-time
operation takes some time to perform and increases the startup time of the
container.

Building A Container

This section provides useful tips for building containers based on this
baseimage.

Selecting Baseimage Tag

Properly select the baseimage tag to use. For a better control and prevent
breaking your container, use a tag for an exact version of the baseimage
(e.g. alpine-3.6-v2.0.0). Using the latest version of the baseimage
(alpine-3.6) is not recommended, since automatic upgrades between major
versions will probably break your container build/execution.

Referencing Linux User/Group

Reference the Linux user/group under which the application is running by its ID
(USER_ID/GROUP_ID) instead of its name. Name could change in different
baseimage versions while the ID won't.

Default Configuration Files

Default configuration files should be stored in /defaults in the container.

Adding/Removing Packages

To add or remove packages, use the helpers add-pkg and del-pkg provided by
this baseimage. To minimze the size of the container, these tools perform
proper cleanup and make sure that no useless files are left after an addition
or removal of packages.

Also, when packages need to be added temporarily, use the --virtual NAME
parameter. This allows installing missing packages and then remove them
easily using the provided NAME (no need to repeat given packages). Note that
if a specified package is already installed, it will be ignored and will not be
removed automatically.

Here is an example of a command that could be added to Dockerfile to compile
a project:

RUN \
    add-pkg --virtual build-dependencies build-base cmake git && \
    # Compile your project here...
    git clone https://myproject.com/myproject.git
    ... && \
    del-pkg build-dependencies

Supposing that, in the example above, the git package is already installed
when the call to add-pk is performed, running del-pkg build-dependencies
doesn't remove it.

Modifying Files With Sed

sed is a useful tool and is often used in container builds to modify files.
However, one downside of this method is that there is no easy way to determine
if sed actually modified the file or not.

It's for this reason that the baseimage includes a helper that gives sed a
"patch-like" behavior: if the application of a sed expression results in no
change on the target file, then an error is reported. This helper is named
sed-patch and has the following usage:

sed-patch [SED_OPT]... SED_EXPRESSION FILE

Note that the sed option -i (edit files in place) is already supplied by the
helper.

It can be used in Dockerfile, for example, like this:

RUN sed-patch 's/Replace this/By this/' /etc/myfile

If running this sed expression doesn't bring any change to /etc/myfiles, the
command fails and thus, the Docker build also.

Modifying Baseimage Content

Try to minimize modifications to files provided by the baseimage. This
minimizes to risk of breaking your container after using a new baseimage
version.

Application's Data

Applications often needs to write configuration, data, logs, etc. Always
make sure they are all written under /config. This directory is a volume
intended to be mapped to a folder on the host. The goal is to write stuff
outside the container and keep these data persistent.

A lot of applications use the environment variables defined in the
XDG Base Directory Specification to determine where to store
various data. The baseimage sets these variables so they all fall under
/config/:

  • XDG_DATA_HOME=/config/xdg/data
  • XDG_CONFIG_HOME=/config/xdg/config
  • XDG_CACHE_HOME=/config/xdg/cache

$HOME Environment Variable

The application is run under a user having its own UID. This user can't be used
to login with, has no password, no valid login shell and no home directory. It
is effectively a kind of user used by daemons.

Thus, by default, the $HOME environment variable is not set. While this
should be fine in most case, some applications may expect the $HOME
environment variable to be set (since normally the application is run by a
logged user) and may not behave correctly otherwise.

To make the application happy, the home directory can be set at the beginning
of the startapp.sh script:

export HOME=/config

Adjust the location of the home directory to fit your needs. However, if the
application uses the home directory to write stuff, make sure it is done in a
volume mapped to the host (e.g. /config),

Note that the same technique can be used by services, by exporting the home
directory into their run script.

Service Dependencies

When running multiple services, service srvB may need to start only after
service SrvA.

Service dependencies are defined by creating a regular file in the service's
directory, its name being the name of the dependent service with the .dep
extension. For example, touching the file:

/etc/services.d/srvB/srvA.dep

indicates that service srvB depends on service srvA.

Service Readiness

By default, a service is considered ready when the supervisor successfully
forked and executed the daemon. However, some daemons do a lot of
initialization work before they're actually ready to serve.

Hopefully, the S6 supervisor supports service startup notifications. This is
a simple mechanism allowing daemons to notify the supervisor when they are
ready to serve.

While support for this mechanism can be implemented natively in the daemon, the
use of the s6-notifyoncheck program makes it possible for services to use the
S6 notification mechanism with any daemon.

Application Icon

A picture of your application can be added to the image. This picture is
displayed in the WEB interface's navigation bar. Also, multiple favicons are
generated, supporting all browsers and platforms.

Add the following command to your Dockerfile, with the proper URL pointing to
your master icon: The master icon should be a square PNG image with a size of
at least 260x260 for optimal results.

# Generate and install favicons.
RUN \
    APP_ICON_URL=https://github.com/jlesage/docker-templates/raw/master/jlesage/images/generic-app-icon.png && \
    install_app_icon.sh "$APP_ICON_URL"

Favicons are generated by RealFaviconGenerator. You can tweak yourself their
display with the following method:

  • Generate favicons yourself with RealFaviconGenerator.
    • Set the path to /images/icons/.
    • Enable versioning and set it to v=ICON_VERSION.
  • At the installation page, choose the Node CLI tab.
  • Copy the content of faviconDescription.json.
  • Minify the JSON using an online JSON minifier.
    • Before running the minifier, modify the masterPicture field to
      /opt/novnc/images/icons/master_icon.png.
  • Copy-paste the result in your Dockerfile. It will be passed to the
    install script.
  • Your Dockerfile should have something like:
# Generate and install favicons.
RUN \
    APP_ICON_URL=https://github.com/jlesage/docker-templates/raw/master/jlesage/images/generic-app-icon.png && \
    APP_ICON_DESC='{"masterPicture":"/opt/novnc/images/icons/master_icon.png","iconsPath":"/images/icons/","design":{"ios":{"pictureAspect":"backgroundAndMargin","backgroundColor":"#ffffff","margin":"14%","assets":{"ios6AndPriorIcons":false,"ios7AndLaterIcons":false,"precomposedIcons":false,"declareOnlyDefaultIcon":true}},"desktopBrowser":{},"windows":{"pictureAspect":"noChange","backgroundColor":"#2d89ef","onConflict":"override","assets":{"windows80Ie10Tile":false,"windows10Ie11EdgeTiles":{"small":false,"medium":true,"big":false,"rectangle":false}}},"androidChrome":{"pictureAspect":"noChange","themeColor":"#ffffff","manifest":{"display":"standalone","orientation":"notSet","onConflict":"override","declared":true},"assets":{"legacyIcon":false,"lowResolutionIcons":false}},"safariPinnedTab":{"pictureAspect":"silhouette","themeColor":"#5bbad5"}},"settings":{"scalingAlgorithm":"Mitchell","errorOnImageTooSmall":false},"versioning":{"paramName":"v","paramValue":"ICON_VERSION"}}' && \
    install_app_icon.sh "$APP_ICON_URL" "$APP_ICON_DESC"

Maximizing Only the Main Window

By default, the application's window is maximized and decorations are hidden.
However, when the application has multiple windows, this behavior may need to
be restricted only to the main one.

This can be achieved by matching on more window parameters: class, name, role,
title and type. By default, only the type parameter is used and must equal to
normal.

To find all parameters of the main window:

  • While the application is running and the main window is focused, login to
    the container.
    docker exec -ti [container name or id] sh
    
  • Execute obxprop --root | grep "^_NET_ACTIVE_WINDOW". The output will look
    like:
    _NET_ACTIVE_WINDOW(WINDOW) = 16777220
    
  • Using this ID, show the parameters by executing
    obxprop --id [MAIN WINDOW ID] | grep "^_OB_APP". The output will look
    like:
    _OB_APP_TYPE(UTF8_STRING) = "normal"
    _OB_APP_CLASS(UTF8_STRING) = "Google-chrome"
    _OB_APP_NAME(UTF8_STRING) = "google-chrome"
    _OB_APP_ROLE(UTF8_STRING) =
    _OB_APP_TITLE(UTF8_STRING) = "Google Chrome"
    

Finally, in the Dockerfile of your container, modify the configuration file of
openbox (located at /etc/xdg/openbox/rc.xml) to apply window restriction.
Usually, specifying the window's title is enough.

set-patch 's/<application type="normal">/<application type="normal" title="Google Chrome">/' /etc/xdg/openbox/rc.xml

See the openbox's documentation for more details: http://openbox.org/wiki/Help:Applications

S6 Overlay Documentation

  • Make sure to read the S6 overlay documentation. It contains information
    that can help building your image. For example, the S6 overlay allows you to
    easily add initialization scripts and services.
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