mariadb: Make user compatible with official image

UID 27 was introduced in commit a8d87d25 came from the upstream Fedora based Dockerfile. But the official upstream images are ubuntu based, which use 999. Hence switching from that to our image with an existing data volume fails to start up.

At this point we can still break our promise to "never change 27". Avoid repeating the value by introducing an ARG, with a big comment.

Also add some documentation how to migrate from a setup where the data files are not owned by UID 999.

See https://issues.redhat.com/browse/HUM-98

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The first push here with just the test failed in CI, reproducing the failure that I saw in production.