New Dockerfile class (with tests).
Added appendAll and enabled method chaining.
Ensure that Dockerfile instructions are always plain Strings (e.g. not GStrings)
Use Dockerfile class, unset MAINTAINER if undef

- Use new Dockerfile class in DockerTask.buildDockerfile
- New DockerTask.buildDockerfile test & test cleanup
- Don't set MAINTAINER if undefined
- entryPoint and defaultCommand can be set as property or via method call.