Print relevant variables to logs as a final step

[choldgraf]

Background and proposal

Context
A common workflow for this action is:

1. Edit an environment file
2. Merge into main
3. Wait for the action to build the image to complete
4. Look at its logs to figure out the SHA for the new image (and its location)

Users are probably looking for the information in step 4, but it's a bit hard to find for people who don't know where to look. For example, you need to know to look for the "Update jupyter dependencies with repo2docker" section, and look to the bottom for the SHA. But what if somebody is using RStudio and not Jupyter? What if somebody doesn't realize that this is not only updating the dependencies but also pushing a new image? They may not know to click there.

Proposed solution

I think that we should add a final step to our action called something like "Print important information about new image".
This would print all of the necessary information a person might want in order to update their image. For example, the location of the registry, the new image tag, a single copy/pasteable string to put into the Configurator, etc.

Implementation guide and constraints

Ideally this is something that we could put into the repo2docker action, but I'm not sure how feasible that is. I think we want these logs to be as easy to find as possible, and if they're embedded as a part of the repo2docker action logs it might be hard to find nonetheless.

An even cooler option (if it's possible) would be to do something similar to the "Test this on Binder" action, and have it post a comment to a PR like:

Finished building user image 🎉
If you merge this PR, the image will be here: `<URL to image on registry>`
The image name and tag will be: `<image-name>:<hash>`

Updates and ongoing work

No response