Based on asssaf/rpi-arch-rootfs and adding docker, btrfs-progs and aufs-util packages.
Also enables the services by default, including docker listening on a TCP socket.
Dockerfile available at https://github.com/asssaf/rpi-qemu-docker/blob/master/rpi-arch-docker/Dockerfile
This image can be exported to a tgz and then converted to an img (using asssaf/rpi-qemu-arch-root) which can be run using QEMU or directly on the Pi.
Requires a kernel with ARM, Raspberry Pi and docker support (can be generated using asssaf/rpi-qemu-kernel-build).