fix: stop the job's docker socket from becoming a directory (#1215)

Fixes https://gitea.com/gitea/runner/issues/1213

Fix the DooD regression that mounts `/var/run/docker.sock` as a directory. Keep the Docker proxy available through job and post steps. Clean stale resources before opening it, then remove containers before their networks and volumes during teardown.

Use a unique filesystem probe and preserve socket ownership. Fall back to direct access when proxying is unsupported. Preserve exec output and clean up active streams and failed starts.

Add a real Docker job test for mounted socket access, post steps and resource cleanup.

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Co-authored-by: silverwind <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1215
Reviewed-by: silverwind <[email protected]>
Co-authored-by: Zettat123 <[email protected]>

README.md

@@ -275,7 +275,7 @@ volumes:
   - ${GITEA_DOCKER_WORKSPACE:-.}/data:/app/data
 ```
 
-Containers, networks and volumes a job creates through the socket are removed when the job ends. Both apply to jobs in containers, on the host `.` already works.
+Linux container jobs use a Docker proxy when socket sharing and permissions allow it. Other setups use the daemon socket directly. The proxy removes containers, networks and volumes after post steps and the completed hook. Named volumes created through it are job-scoped. To retain resources, mount the daemon socket explicitly in `container.options`. Host jobs use their existing Docker access, where `.` works.
 
 #### Proxy