TestS3Minio brings up a minio container via the fstest/testserver
framework, which exec's bash init.d scripts that shell out to docker.
This is not available on all platforms - Windows has no POSIX shell to
run the scripts, and macOS CI runners have no docker daemon - which
caused the build to fail there.
Add testy.SkipUnlessDocker to detect whether the framework can run and
skip the test when it cannot.
For unknown reasons the precision of modification times of directories
on the CI is > 15mS compared to files which are 100nS. The tests
work fine when run in Virtualbox though so I conjecture this is
something to do with the file system used there.
Nick Craig-Wood