Add support for HTTPS & Unix sockets

Respect the DOCKER_HOST and DOCKER_CERT_PATH environment variables and
connect to Docker instances over HTTPS or via Unix sockets.
Wouldn't it be empty if there is no `<dockerHost>` specified in the configuration in the pom.xml?

If it is empty, then we just fall back to whatever the default is based on `DefaultDockerClient.fromEnv()`.