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
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.