Relocated the scope for common docker settings

I found myself forgetting the configuration in `dockerExposedPorts in Docker` and so I thought that the `dockerExposedPorts` and `dockerExposedVolumes` shouldn't need the configuration provided. The other commonly used keys do not e.g. `dockerBaseImage`.
Merge pull request #392 from sbt/scope-removal-docker

Relocated the scope for common docker settings