Initial support for Dockerfile that builds the Nexus OSS with NPM Registry

This commit adds the initial Dockerfile that publishes an image of the
Nexus Server with the Registry. The following is configured:

* Exposes the server on port 8081
* Exposes a volumen at /opt/sonatype-work for the storage
* Runs as "root" for now.

*	new file:   Dockerfile
- Initial docker file.