Allow hubs to maintain their own user image

[yuvipanda]

For most of the hubs hosted on our cluster (in *.pilot.2i2c.cloud, and *.cloudbank.2i2c.cloud), we use this user image. This is a bit generic, and has things many people want. But customizing this on a per-hub basis doesn't work very well!

1. Version conflicts will happen quickly and cause big problems
2. Any environment updates require intervention from 2i2c engineers
3. If we install additional languages, becomes confusing for users of hubs that don't need it.

So we need a solution that's as close to binder as possible, in terms of empowring our hub admins. It should:

1. Give hub admins complete control over the user environment
2. ... in a way that lets them iterate and test the environment with standard tools (in this case, mybinder.org)
3. ... without having to deeply understand how all the pieces (repo2docker, docker, mybinder, binderhub, jupyterhub, etc) fit together
4. ... with minimal intervention from 2i2c staff.

Potential solution

For the peddie hub, @sawula requested the py5 package. This adds a new kernel, and also requires Java. Instead of adding it to default image, I've instead:

• Forked the binderized example repo to https://github.com/2i2c-org/peddie-image (Thanks to Add py5, a Python Processing library  #479 (comment))
• Setup repo2docker-action to push to our quay.io account
• Setup a different action to comment with a 'test this in binder' link for each PR
• Add docs on how to update your hub to use a new image + tag with the JupyterHub Configurator

There's lots to be improved here, but I think this is a good start on making this process easier.

Related: #312

[yuvipanda]

Pre-existing very helpful feedback about the configurator: #329 (comment)

[damianavila]

This is great @yuvipanda!
I really like the idea "test this in binder"... it is super important the users do not need to deal with local setups to test their changes.
Maybe one future improvement would be a way to automatically update the image from the very same image repository, so they do not need to go through the configurator step... although the configurator path is more than enough for now, IMHO.

[choldgraf]

Could we consider this a duplicate of #631 ? I think that setting up the "test this in binder" would be cool but could be tracked as a separate enhancement rather than a "must have" feature.

[GeorgianaElena]

@choldgraf, I opened an issue about the test this in binder feature in the new user image template repository.

I think this was the only thing was missing from #631. So, I think we could potentially close this one and leave the other issue open (since it's is more "closer" to where the change should happen).

What do you all think?

[choldgraf]

perfect - that makes sense to me! let's close this and track it in #631 thanks Georgiana :-)