Fix docker CI build size

[andrewfayres]

Summary

Made some improvements to the docker CI to work around some space limitations. Biggest difference is adding an action which cleans up a lot of disk space before we ever run anything. This was news to me but apparently the runners come loaded with a lot of things rarely used like android sdk, haskell, and docker images which use up a significant amount of room. We're removing most of that to reclaim space.

Motivation & Context

Fixes #75

Approach

Reclaimed space by removing some preloaded stuff from the runners, building cpu-only torch (no point in building gpu since the runner is cpu and we aren't publishing), changed dockerfile a bit to squash some layering and save more space.

Testing Plan

• Unit tests
• Integration tests
• e2e / smoke test
• Manual steps: python -m app --help

Documentation

• Docstrings updated
• User docs / README updated
• CHANGELOG entry

Checklist

• Code formatted (Ruff) → ruff format --check
• Lint passes (Ruff) → ruff check .
• Types pass (mypy/pyright) → mypy src
• Tests pass (pytest) → pytest -q

• Backward compatibility considered
• Adequate comments for tricky parts
• CI green

Risk & Rollback Plan

Probably not needed in the beginning

Notes for Reviewers

[ScSteffen]

lgtm. Also wasn't aware of the default payload in the CI runs. Good learning for other projects too