Improve support for running lint/test on linux

When running the docker containers to run the lint/test suites from a linux host there was an undocumented requirement that the somebody uid/gid inside the container had to match the uid/gid of the host system. This is not always the case.

Improve support by adding an entrypoint script that, when running the container as root, will modify the somebody uid/gid inside the container to match the uid/gid of the mounted filesystem.