capd/baseimage repository overview

⁠docker-baseimage

Yet another Docker⁠ base image to make it easier to connect to your running Docker instances.

⁠With a Little Help from Phusion

Based on Phusion's excellent baseimage-docker⁠, this Docker base-image features all of the awesome features you've grown to love like working CRON jobs, a SSH server, a lightweight working init system (that actually works!), and more.

⁠...But Even Better

Phusion's baseimage project is currently debating how best to allow users to connect to running Docker instances. They provide the OpenSSH daemon but no real way to easily add your own keys and connect once a Docker image has already been built.

In the interest of modularity, our Docker base-image provides two different ways to add SSH keys to Docker instances without any build-time tweaks and without requiring external tools like nsenter.

⁠Add SSH Keys with Environment Variables

The first option we provide is to specify SSH keys in an environment variable when starting your Docker instance:

sudo docker run -d -e SSH_KEYS="$(cat ~/.authorized_keys)" rfkrocktk/baseimage

On startup of the container, it will add each SSH key found in the SSH_KEYS environment variable to the $HOME/.ssh/authorized_keys file inside of the container. You'll then be able to quickly and easily SSH into the Docker instance using your key.

⁠Supported Environment Variables

⁠Add SSH Keys with a Docker Volume

This is an even easier way to maintain a Docker instance's authorized_keys file. In our base-image, we've created a directory for each user called $HOME/.ssh/authorized_keys.d/ which contains the authorized_keys file which is then symlinked back to $HOME/.ssh/authorized_keys. What this means is that you can easily maintain the authorized_keys file outside of the Docker instance by using a Docker volume:

sudo docker run -d --name "dockerduck" -v /host/path/to/dockerduck-ssh-keys:/root/.ssh/authorized_keys.d \
    rfkrocktk/baseimage

Now, simply open /host/path/to/dockerduck-ssh-keys/authorized_keys and add your SSH public keys there:

sudo cat ~/.ssh/authorized_keys > /host/path/to/dockerduck-ssh-keys/authorized_keys

Done! Now just connect to your Docker instance!

Note: environment variables will always overwrite $HOME/.ssh/authorized_keys, so don't mix and match. Use one or the other, not both.

⁠Connecting to Your Docker Instance

Now that you've added your keys to your Docker instance, you need to know its internal IP address to connect to it. You can use the following command to see your Docker instance IP addresses:

sudo docker inspect -f "{{.NetworkSettings.IPAddress}}" dockerduck

In my case, the IP address is 172.17.0.31, therefore:

ssh [email protected]

Note, you probably have to enable agent-forwarding if you're connecting to a Docker instance hosted on a remote server, as your agent only runs on your local machine and is terminated on your first hop.