GitHub Actions are documented to recognize the exit code 78 as neither passing nor failing:
When an action returns this exit status, GitHub terminates all concurrently running actions and prevents any future actions from starting. The associated check run shows a
neutralstatus, and the overall check suite will have a status of
successas long as there were no failed or cancelled actions.
In practice, when a process exits with that code, GitHub reports the Action and containing Workflow as “failed.” The logs end with the following line:
##[error]Docker run failed with exit code 78
This was working as expected until my project migrated to the latest beta version of GitHub Actions, so this may be a regression in that release.