Push challenge image as part of CI

[robwaz]

If we are building the challenge image every night, we should push it to a public docker hub account so that others can reference it for dev/testing/rebasing purposes. In the long-term this may not be necessary, but as we transition to nix, it could be useful to dojo builders.

[ConnorNelson]

I agree.

As a note, we currently only build the minimal image every night.

This is what should happen:

• Move ./challenge into public repo github.com/pwncollege/challenge-legacy
• GitHub action to build/push to pwncollege/challenge-legacy:latest on push to the GitHub repo; bonus points for not-rebuilding intermediate layers for speed, but probably not critical
• Do the same for a pwncollege/challenge-legacy:latest-slim
• Change this repo so that we pull pwncollege/challenge-legacy:latest-slim and re-tag it as pwncollege/challenge-legacy:latest before test-cases run (to make sure CI doesn't break / fix CI)
• Change this repo to default to pwncollege/challenge-legacy:latest for image to keep current dojos working
• Transition all our dojos so they explicitly depend on pwncollege/challenge-legacy:latest as the challenge image
• Give other people time to do the same, and then migrate to a default image of ubuntu:24.04, or maybe require the image to be set (a little bit less sure on this one)

[ConnorNelson]

Quick attempt at beginning this process: https://github.com/pwncollege/challenge-legacy