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.
Nick Craig-Wood
9c9fbebf7f