feat: add GITEA_DOCKER_WORKSPACE and container cleanup (#1204)

1. Add `GITEA_DOCKER_WORKSPACE` which holds the workspace path as the daemon sees it, enabling `${GITEA_DOCKER_WORKSPACE:-.}/data:/app/data` in a compose file without having to resort to `bind_workdir` (which causes much more problems like breaking `actions/cache` because of unstable workspace paths).
2. Add container/network/volume cleanup for containers started within jobs, for example via `docker compose` inside a job. It works by running a lightweight docker socket proxy and injecting a `com.gitea.runner.job` label into every container creation and that label is used to remove containers started by that job at the end. Perf impact of this is near-zero.

Docs: https://gitea.com/gitea/docs/pulls/535

Assisted by Claude (Fable 5.1).
Co-authored-by: bircni <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1204
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

act/container/docker_run_test.go

@@ -801,6 +801,20 @@ func TestContainerInfoFromInspect(t *testing.T) {
 		assert.Equal(t, "abc123", info.ID)
 		assert.Equal(t, HealthNone, info.Health)
 	})
+
+	t.Run("reports the mount sources by container path", func(t *testing.T) {
+		info := containerInfoFromInspect(container.InspectResponse{
+			Mounts: []container.MountPoint{
+				{Type: mount.TypeVolume, Name: "job", Source: "/var/lib/docker/volumes/job/_data", Destination: "/workspace/owner/repo"},
+				{Type: mount.TypeBind, Source: "/var/run/docker.sock", Destination: "/var/run/docker.sock"},
+			},
+		})
+
+		assert.Equal(t, map[string]string{
+			"/workspace/owner/repo": "/var/lib/docker/volumes/job/_data",
+			"/var/run/docker.sock":  "/var/run/docker.sock",
+		}, info.Mounts)
+	})
 }
 
 func TestMergeContainerConfigsVolumesReplaceRunnerMounts(t *testing.T) {