Allow docker publish to add multiple tags

Fully backwards compatible.

- Added custom_tags string array to allow multiple tags to be applied.  If both custom_tag and custom_tags are specified they will be concatenated.  
- Only tags that are created are pushed to the repository rather than the whole repo
- Docs PR is incoming.

Any disadvantages to do it as a single call to "docker rmi tag1 tag2 tag3..." instead of once per tag.?