binder startup often times out

[bsipocz]

We should follow suggestions from here. Not sure whether it's possible to cut back on installing dependencies, all of the listed ones are used in at least one notebook.

Adding the environment file from a separate repo could be a solution though.

https://discourse.jupyter.org/t/how-to-reduce-mybinder-org-repository-startup-time/4956

[bsipocz]

Another, probably somewhat questionable, workaround is to trigger a binder click/build from cron to bump the chance of an image is already available at the time an actual user clicks on the link.