fix: skip step-checker in container jobs where _diag is not accessible

[adityamaru]

Problem

The step-checker feature was causing warnings and preventing sticky disk commits in container jobs:

Warning: Unable to check for previous step failures: _diag directory not found at /__w/web/web/_diag
Warning: Skipping sticky disk commit due to ambiguity in failure detection

Root Cause:

• Container jobs mount the workspace at /__w/ inside the container
• The _diag directory exists on the host at /home/runner/_diag but is not mounted into containers
• The step-checker tried to access _diag and failed, returning an error
• This prevented sticky disk commits from happening

Impact:

• Affecting container jobs since the step-checker was deployed
• Only affects jobs with container: in workflow config

Solution

Add container detection by checking /proc/1/cgroup for docker/containerd. When running inside a container, skip the step-checker gracefully since _diag is not accessible.

No changes to existing path detection logic - only adds the container check at the beginning.

Testing

• ✅ Build successful
• ✅ All tests pass
• ✅ Container jobs will now skip step-checker and allow sticky disk commits
• ✅ Non-container jobs continue to work exactly as before

Related

This is the same fix applied to setup-docker-builder in useblacksmith/setup-docker-builder#53

---

Note

Skip step-checker when running in containers by detecting container environments; update CI to pass github_token to Buf setup.

• Step checker (src/step-checker.ts, dist/post/index.js):
  • Add container environment detection (/.dockerenv, /proc/1/cgroup, cwd prefix /__w/).
  • If in container, skip _diag lookup and return no failures (allows commits to proceed).
• CI (.github/workflows/build.yaml):
  • Provide github_token to bufbuild/buf-setup-action@v1.

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