allow target path to be configurable

[aplamada]

This update is handy in case one is interested to configure the target path provided via --target-repo-dir for the Dockerfile usecase.

The ultimate goal might be to update the https://mybinder.readthedocs.io/en/latest/tutorials/dockerfile.html and potentially repo2docker such that this becomes a requirement of the Dockerfile (a warning in case it is not satisfied).