Problem matcher fails in self-hosted runner due to unexpected GITHUB_WORKSPACE

I have an action whose problem matcher seems to only fail when run self-hosted. The file is getting dropped due to not being under the workspace root:

- File "./src/demo.v", line 5, characters 7-13:
  ##[debug]Dropping file value '/home/ubuntu/coq-ci/actions-runner/_work/coq-github-ci-test/coq-github-ci-test/src/demo.v'. Path is not under the workflow repo.

I’m not exactly sure how to debug this since I don’t know what the workflow repo is, but I was surprised to find that while RUNNER_WORKSPACE is /home/ubuntu/coq-ci/actions-runner/_work/coq-github-ci-test like I’d expect, GITHUB_WORKSPACE is /github/workspace when my action runs (even though it was the same as RUNNER_WORKSPACE during actions/checkout@v2).

The full logs are at

Hi @tchajed,
As you are using a docker container action ( tchajed/docker-coq-action@no-opam ), the Docker container filesystem is not the same as the runner. The RUNNER_WORKSPACE is mapping to GITHUB_WORKSPACE inside the action container.

You need to use GITHUB_WORKSPACE inside docker container instead of RUNNER_WORKSPACE .