Adding support for TLS DOCKER_HOST

Workarounds courtesy of
https://github.com/fsouza/go-dockerclient/issues/173. They can likely be
removed after better support for TLS docker connections is added to
go-dockerclient.
Not super happy about this check being done by port - why not set it based on `os.Getenv("DOCKER_TLS_VERIFY")`?  Or am I missing something?