Refactor docker setup

[LiquidPL]

Resolves #126.

This PR cleans up all the Docker and Docker Compose files used by oioioi.

Notable changes:

• reworked Dockerfile to use multi-stage builds, now the resulting image doesn't have any leftovers from the build process,
• the app is now running inside a virtualenv,
• updated django-supervisor to work with Python 3 properly, and fixed a bug which would sometimes cause it to break in virtualenvs, currently it's hosted in my fork at https://github.com/LiquidPL/django-supervisor, however I can move it to the sio2project org if it's preferred that way,
• it is now possible to build a smaller, production-ready image that doesn't include apt installed compilers,
• removed sandboxes from the development image, as mounting a volume/directory to the already existing filetracker database in the image would lead to confusing behaviour (and additionally, the sandboxes would be pointlessly duplicated in every image),
• added instructions on how to install the sandboxes on a machine without internet connection
• various settings (database, rabbitmq, judging) that were previously hardcoded are now configurable via environment variables (with original hardcoded values being now configured in compose files)
• removed duplicated docker container init scripts
• moved all scripts that are not being run on the host machine to a separate scripts/ directory
• removed duplicated configuration in docker-compose-selenium.yml
• changed docker-compose configs to use an external sioworkers image

Some additional things to consider:

• Is the selenium test suite actively maintained? I've made sure that it at least starts up properly, but from what I've seen all the tests fail and the suite hasn't been touched in a few years.
• Why is the development server being started as a separate command instead of being managed in supervisor? I haven't seen anything that would suggest it can't be run that way, and this way we could simplify the docker-compose commands by removing extra/sample_configs/basic_settings_noserver.py.
• This PR still lacks scripts and GitHub Actions workflows to build and publish the Docker images, as I'd prefer to discuss where to publish those images beforehand.
• There's also two PRs for sioworkers at Fix Sio2Jail executor failing on Python 3 sioworkers#11, Add Dockerfile and CI workflows building Docker images on new releases sioworkers#12.
• I'd also like to remove the local installation of sioworkers from the image (at least for production builds), however I'm not sure if it's being used anywhere.

[LiquidPL]

It fails because there's no sio2project/sioworkers image published yet.

[tudny]

Okay, my mistake. Sorry

[LiquidPL]

I've pushed an image to liquidpl/sioworkers:test. You can change the tag in https://github.com/gs-hackathon-this-is-fine/oioioi-docker/blob/docker/docker-compose-dev.yml#L46 and run the tests again.

[twalen]

I prefer to have this repo in sio2project organization

[LiquidPL]

I've opened a pull request to bring in my changes: sio2project/django-supervisor#1.

[twalen]

I've merged your PR to django-supervisor

[LiquidPL]

I've switched django-supervisor to use the sio2project fork.

I'm going to convert this PR to a draft for the time being, since at this moment it's not going to work, without the relevant sioworkers PR being merged, and the Docker images published.

[LiquidPL]

Apologies for not finishing up this pull request, I will get to it very soon. However, there's one problem, mainly that the sioworkers docker image seems to be not public, ie. the packages section shows one available package, but after clicking on it, the resulting search shows no results:

I'm guessing it's just a matter of switching the package to be publicly available.