Docker and context path

It is not clear if our Docker support works with a context path configured in org.sonatype.nexus.cfg

The following steps need to be taken

• test if it works or not
• decide if we want it to work or not (if thats even possible)
• update docker chapter in documentation accordingly
• update section about configuring context path accordingly
• answer http://stackoverflow.com/questions/33021350/trouble-connecting-to-docker-registry-stored-on-nexus-3-preview-on-azure-vm