Separate and simplify PyDis specific usage

[jb3]

This PR makes a start at decoupling the specific setup of snekbox PyDis uses and snekbox as a standalone application.

This is done by building a basic snekbox application that supports evaluation using only the system version of Python.

We then have a new separate Dockerfile.pydis that uses the pre-built versions from python-discord/python-builds.

An integration specific image using this Dockerfile is built for integration tests to mirror the current behaviour.

[coveralls]

coverage: 88.265%. remained the same
when pulling d08f0bc on jb3/pydis-version-separation
into 28c95c8 on main.

[MarkKoz]

What's the purpose of hosting a registry in our job?

[jb3]

Having a registry allows us to use the newly built ghcr.io/python-discord/snekbox image for our snekbox-pydis build (you will see later that images are pushed to localhost:5000).

https://stackoverflow.com/a/63927832

There is probably some combination of cache-to/cache-from that may also work here but creating a local registry is a 3 second operation that then ensure we are using the right images and there will be no attempt made to pull images if the cache operations fail for whatever reason.

[MarkKoz]

I forgot that steps within the same job don't share the cache. But I thought that's what the existing cache-to/from already solve.

[MarkKoz]

What does the help output look like now?

[jb3]

Old:

root@snekbox-6c47cc4bf5-9gchm:/snekbox# python -m snekbox --help
usage: snekbox [-h] code [nsjail_args ...] [--- py_args ...]

positional arguments:
  code         the Python code to evaluate
  nsjail_args  override configured NsJail options (default: [])
  py_args      arguments to pass to the Python process (default: ['-c'])

options:
  -h, --help   show this help message and exit

New:

root@a7cdb21501e0 /snekbox
❯ python -m snekbox --help
usage: snekbox [-h] code [nsjail_args ...] [--- py_args ...]

positional arguments:
  code                  the Python code to evaluate

options:
  -h, --help            show this help message and exit
  --nsjail-args [NSJAIL_ARGS ...]
                        override configured NsJail options (default: []) (default: [])
  --py-args [PY_ARGS ...]
                        arguments to pass to the Python process (default: ['-c']) (default: ['-c'])

I can try find a solution that brings them closer together, they are functionally the same (hence no updates in the test cases) but the help is presented differently.

[MarkKoz]

Why is the default shown twice now?

[MarkKoz]

They're now options rather than positional arguments. That's confusing.

[jb3]

Why is the default shown twice now?

We hard-coded it previously but now switching it from store_const appears to display it again, we can remove our hardcoding.

They're now options rather than positional arguments. That's confusing.

Yeah, I'll be honest I'd just updated it to maintain functionality because argparse in 3.14 broke it, hadn't looked at how it changed the help command.

I'll have a play around to see if I can maintain behaviour on 3.14 and keep the help command useful.