Add checkout-with-cache composite action

[benstpierre]

Summary

This is a small proposal based on a helper action we have been using at Galatea to speed up large repository checkouts on RunsOn runners. It adds a composite action at checkout-with-cache/ that restores a cached repository .git snapshot from RunsOn EFS before running actions/checkout, then updates and rotates the cache afterward.

I am not 100% sure whether this repository is the right final home for it, so I opened this as a draft. If you would rather have this live in a tiny dedicated repo like runs-on/checkout-with-cache, that may actually be better for runtime performance because checkout acceleration should not require downloading a larger action repo first. Happy to reshape this however you prefer.

Why

For large repos using fetch-depth: 0, checkout can dominate job startup time. In one Galatea repo (galateatech/waste-coordinator), we saw roughly:

• plain actions/checkout@v6 with fetch-depth: 0: about 34s
• EFS restore plus checkout delta fetch: about 6s total

This felt like something worth proposing upstream because the useful bit is really the RunsOn EFS/cache behavior, not anything Galatea-specific.

Usage

steps:
  - uses: runs-on/action/checkout-with-cache@v2
    with:
      encryption_key: ${{ secrets.CHECKOUT_CACHE_KEY }}

The action wraps actions/checkout@v6, defaults to /mnt/efs/${GITHUB_REPOSITORY}/.git, stores files as repository-<sha>-<timestamp>.tar, and optionally encrypts them with openssl enc -aes-256-cbc -pbkdf2.

Safety / behavior

• only enables cache behavior on RunsOn Linux runners with EFS available
• cache misses and restore failures fall back to normal checkout
• writes are skipped on pull request events
• writes default to the repository default branch
• persist-credentials defaults to false for actions/checkout
• checkout extraheaders are removed before saving .git
• old cache archives are rotated by timestamp

Validation

• bash -n checkout-with-cache/*.sh
• shellcheck checkout-with-cache/*.sh
• YAML parse check
• local plain tar save/restore smoke test
• local encrypted tar save/restore smoke test

Thanks for taking a look. We found this really helpful internally, and I figured it might be useful to other RunsOn users too if you like the shape.