Record resolved reference in container image label

[betatim]

Bug description

If you run the following command (not explicitly providing a --ref):

repo2docker --no-build https://github.com/bokeh/bokeh-notebooks

will lead to the following labels being added to the container image:

LABEL repo2docker.ref="None"
LABEL repo2docker.repo="https://github.com/bokeh/bokeh-notebooks/"
LABEL repo2docker.version="0.11.0+179.g19c698e.dirty"

Expected behaviour

We should record the "resolved ref" instead of None. For Git based content providers this would be the SHA1 of the commit we are building.

This was noticed during #977 which has a quick back and forth on whether this behaviour is "as designed" or a bug. Conclusion was that this is a bug.