Dockerfiles for Mercurial 5.4

This provides Docker definitions for extension tests on Mercurial 5.4

The last changeset helps them build on a system that already has built the other images, hence working around #2. It would not introduce any harm once #2 is fixed and we rebuild everything (no-cache), but we can just ignore it if #2 is fixed.

As usual, the resulting images have already been uploaded to Docker Hub, making the 5.4 runs that are immediately needed possible.