Add Docker file and build it in CI

Supersedes !9 (closed)

As discussed in last meeting, this continues the work in the above merge request and modifies it so that docker images are only pushed from the master branch. Docker images are should however be tested on all branches.

I've added instructions to the README.org, however, I can't test them until an image has been pushed, which won't happen until this branch is merged. But the Docker image I've built locally works (and the one built in this pipeline, but which is not pushed).