fix: align common behaviour with GitHub's runner (#1413)

Closes differences from GitHub's runner that common workflows hit:

- `ACTIONS_STEP_DEBUG` set as a repository variable enables debug output as on GitHub, where a secret of the same name still takes precedence. `runner.debug` and `RUNNER_DEBUG` are now also set inside composite actions, which lost the secret.
- `github.env` and `github.path` hold the file-command paths instead of being empty, so `echo "A=1" >> "${{ github.env }}"` works.
- Remote composite actions only get a pre stage when they nest actions. That stage evaluated `with:` before earlier steps ran, so an input like `${{ fromJSON(steps.meta.outputs.json).v }}` failed the job.
- `ports` on a job container are published, they were ignored while service ports worked.
- Dockerfile action build output reaches the job log. It was logged at debug and dropped, so a failing `RUN` showed only the final error.
- Local Docker actions honor their `.dockerignore`, as `docker build` does.
- Cancelling or timing out a host-mode step sends SIGINT, then SIGTERM after 7.5 seconds and SIGKILL after 2.5 more, as GitHub's runner does, so `trap` handlers and shutdown hooks run.
- `.sh` job hooks fall back to `sh` where bash is missing, and `.ps1` hooks to `powershell` where pwsh is missing, as on GitHub.

_Written by Claude (Opus 5.5)._

Reviewed-on: https://gitea.com/gitea/runner/pulls/1413
Reviewed-by: bircni <[email protected]>
Co-authored-by: silverwind <[email protected]>

docs/host-mode.md

@@ -12,7 +12,7 @@ steps:
 
 - Set the `RUNNER_TRACKING_ID` override in the step's `env` before it starts. Any value different from the job's ID opts out, including an empty string. On Linux and macOS a script can also set it before starting a process.
 - Opted-out processes and descendants that inherit the override survive normal job cleanup and expired output waits. Detach helper output because the runner closes leftover output pipes after a bounded wait.
-- Cancelling or timing out an active step still terminates its process tree.
+- Cancelling or timing out an active step still terminates its process tree, on Unix with SIGINT, then SIGTERM after 7.5 seconds and SIGKILL after 2.5 more.
 - The override does not preserve workspace or temporary files. Store persistent helper files outside those directories and arrange the helper's eventual shutdown.
 
 This follows [actions/runner's tracking ID and orphan cleanup](https://github.com/actions/runner/blob/main/src/Runner.Worker/JobExtension.cs). Container jobs follow their container's lifecycle.