Binderhub: image_prefix broken for DockerHub.

[MridulS]

Bug description

image_prefix for images hosted on DockerHub expects to be of the format image_prefix: <docker-id OR organization-name>/<prefix>- but the recent changes in https://github.com/jupyterhub/binderhub/pull/1269 breaks that. The image_prefix in case of docker hub only has one '/' so it breaks the image name.

In [38]: image_name = 'orgname/repo:HEAD'

# this is new behaviour 
In [39]: image_name.split('/',1)[-1].split(':', 1)
Out[39]: ['repo', 'HEAD']

# this is old behaviour 
In [40]: '/'.join(image_name.split('/')[-2:]).split(':', 1)
Out[40]: ['orgname/repo', 'HEAD'] 

Expected behaviour

DockerHub should work as expected.

Actual behaviour

Getting Failed to get image manifest for ...... errors.

How to reproduce

Go to gesis.mybinder.org and try running a repo twice. (We use DockerHub).

Your personal set up

• OS:
  

• Version(s):
  

  • Full environment

    
    

# paste output of `pip freeze` or `conda list` here

• Configuration

  
  

# jupyterhub_config.py

• Logs

  
  

  # paste relevant logs here, if any
  

[MridulS]

similar behaviour at turing.mybinder.org (the server doesn't launch but the build pods seems to running and pushing to docker hub without any issues, just tried with https://hub.docker.com/r/turingmybinder/binder-prod-networkx-2dnotebooks-0c9816/)

[rappertomate]

The new PR https://github.com/jupyterhub/binderhub/pull/1283 should not let this happen again. See this snippet from the according unit test. Full image name -> image name, tag:
("jupyterhub/k8s-binderhub:0.2.0-a2079a5", "jupyterhub/k8s-binderhub", "0.2.0-a2079a5")