Update the gallery package before building new images

[jtpio]

This PR Implements the idea from #37 (comment) (option 2) so the gallery can be updated automatically.

It also introduces the GALLERY_REPO environment variable to let the users configure where their gallery is located. The full install command for a new installation can then become:

#!/bin/bash
export GALLERY_REPO=git+https://github.com/<your-username>/gallery@master#"egg=tljh-voila-gallery&subdirectory=tljh-voila-gallery"
curl https://raw.githubusercontent.com/jupyterhub/the-littlest-jupyterhub/master/bootstrap/bootstrap.py \
 | sudo python3 - \
   --plugin ${GALLERY_REPO}

@yuvipanda does the JupyterHub service need to be restarted after updating the package?

[jtpio]

This looks like the simplest / easiest way to execute one more command.
If we plan to add more logic to this unit in the future we can consider moving it to a proper script.

[yuvipanda]

hmm, I think this needs to be a separate process, since you'd need to restart the hub process too when this happens.

or, we can split the repo into two, code and data. Thta's the right thing to do long term anyway... we can then unconditionally pull in the data

[jtpio]

what does data mean in that case?

[yuvipanda]

Data would be gallery.yaml

[jtpio]

Then we are back to the question of what happens when the structure of the yaml changes and requires the new code that goes with it.

[yuvipanda]

Ugh, yeah.

But now when the code changes, the files will change but the running processes will not... Immutable deployment problems.

I don't have a clear answer, but if this unblocks you I'm happy to merge as long as we open an issue talking about what is the way out.

[jtpio]

Exactly, this would require a restart.

[jtpio]

As a side note this change also does not cover the case when the hooks have been updated and should be run one more time through the --plugin mechanism of TLJH. For example when adding more custom actions in post_install_hook.

In that case it would probably be better to bootstrap a complete new instance and tear down the old one.