When /dev/ttyACM0 is mounted inside the container, the microbitdev user doesn't have the appropriate permissions to access it.

With the initial compilation issues resolved via lancaster-university/microbit-targets#1 , the Docker based dev environment can now be used to build MicroPython for the micro:bit via:
docker pull ncoghlan/microbit-dev
docker run -it --device=/dev/ttyACM0 --name=microbit ncoghlan/microbit-dev

However, it doesn't currently support pushing the image back to the micro:bit, since the USB Mass Storage interface isn't exposed to the container by default. This problem also affects the output of the tools/makecombinedhex.py script in the micropython repo.

There seem to be two main options available here:

• describe how to map the USB block device interface into the container as well, mount it as a directory, and then copy the built hex files to the micro:bit through the container shell used for building. The downside of this approach is that it increases the complexity of getting the container up and running, as you have to figure out which block device to map into the container (and it may not be possible to get it to work at all on Windows and Mac OS X, where there is a VM running Docker in the mix).
• describe how to map a host filesystem volume into the container, and then use that to copy files to the host, and then use the host to copy them onto the micro:bit. That's a clumsier two step process each time you change the firmware, but has the virtue of being more likely to work on Windows and Mac OS X in addition to Linux hosts.