text: Title: Add labels for repo2docker version and repo 
  Body: it would be handy to add the labels for repo2docker version, repo URL, and ref to images when we build them:
  
  ```docker
  LABEL repo2docker.version="x.y.z"
  LABEL repo2docker.repo="https://..."
  LABEL repo2docker.ref="abc123"
  ```
  
  This records in docker image metadata info about what was built, which could be useful in the future.