There is an issue that the Docker containers cannot modify the
files on KEYMAN_ROOT, since the user running build and tests on
Docker does not have the same gid and uid as the host user starting
the container.
The issue is solved by temporarily changing the uid and gid to
match the container user's uid and gid.
Test-bot: skip
Build-bot: skip
It is now possible to keep the Docker images for building and testing
in a Docker container registry.
This is the first part of changes for #13954. The GitHub actions part is
missing.
Fixes: #13954
Test-bot: skip
Build-bot: skip
This change verifies that `docker buildx` is available and then uses that
for building the images. For that we apparently need the magic line at
the top of the `Dockerfile`s and set the environment variable
`DOCKER_BUILDKIT`. This solves a problem building docker images on
Linux with an older Docker version where the heredocs didn't work.
