Building a Docker image inside a Docker container needs extended
privileges. The default, shared 0xacab worker doesn't support this, so
this job failed for people forking our repo, effectively blocking
merging.

At the same time, up until now we've build Docker images for all
branches. This is unneeded, and uses too much storage space.

This commit fixes both issues in a nice and sane way. Thanks, varac!

Closes #306
Closes #307