fix: matrix-job data races + outputs, leaner offline test suite (#994)

Running the full suite under `-race` (dropping `-short`) exposed pre-existing data races in parallel matrix-job execution, fixed by not sharing mutable state across combinations:

- `containerDaemonSocket()`/`validVolumes()` derive per-job values instead of mutating shared `Config`
- `getWorkflowSecrets` builds a fresh map, `rc.steps()` clones each step, and go-git workdir access is serialized
- every write to a shared `Job`'s result/outputs runs under a per-`Job` lock, each combo interpolating outputs from a pristine snapshot (last wins, as on GitHub)

### Test suite

- capability gates (docker / network / host-tools / Linux) replace the `-short` skips, and the suite runs offline via local fixtures (the artifact flow uses an in-process loopback server, only the docker-action force-pull needs the network)
- drops redundant tests, adds a regression test for https://gitea.com/gitea/runner/issues/981 and a docker-in-docker harness (`make test-dind`)

---
This PR was written with the help of Claude Opus 4.7

Reviewed-on: https://gitea.com/gitea/runner/pulls/994
Reviewed-by: Nicolas <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

act/container/docker_run_test.go

@@ -28,14 +28,10 @@ import (
 )
 
 func TestDocker(t *testing.T) {
-	if testing.Short() {
-		t.Skip("skipping integration test")
-	}
+	requireDocker(t)
 	ctx := context.Background()
 	client, err := GetDockerClient(ctx)
-	if err != nil {
-		t.Skipf("skipping integration test: %v", err)
-	}
+	require.NoError(t, err)
 	defer client.Close()
 
 	dockerBuild := NewDockerBuildExecutor(NewDockerBuildExecutorInput{
@@ -302,6 +298,35 @@ func TestDockerCopyTarStreamErrorInMkdir(t *testing.T) {
 	client.AssertExpectations(t)
 }
 
+// TestDockerCopyToSymlinkPath is a regression test for gitea/runner#981. Most base images
+// symlink /var/run to /run, so copying into /var/run/act traverses that symlink. The broken
+// docker 29.5.1 daemon fails the extraction with "mkdirat var/run: file exists" (fixed in
+// 29.5.2). Running against the daemon shipped in the dind image, this catches a bad bump.
+func TestDockerCopyToSymlinkPath(t *testing.T) {
+	requireDocker(t)
+	ctx := context.Background()
+
+	rc := NewContainer(&NewContainerInput{
+		Image:      "alpine:latest",
+		Entrypoint: []string{"sleep", "30"},
+		Name:       "act-test-symlink-" + time.Now().Format("20060102150405.000000"),
+		AutoRemove: true,
+	})
+	require.NoError(t, rc.Pull(false)(ctx))
+	require.NoError(t, rc.Create(nil, nil)(ctx))
+	require.NoError(t, rc.Start(false)(ctx))
+	t.Cleanup(func() {
+		_ = rc.Remove()(ctx)
+		_ = rc.Close()(ctx)
+	})
+
+	// CopyTarStream first creates the destination directory by extracting a tar at "/",
+	// which makes the daemon mkdir var, then var/run (the symlink), then act — the exact
+	// step that fails on the broken daemon.
+	err := rc.CopyTarStream(ctx, "/var/run/act", &bytes.Buffer{})
+	require.NoError(t, err)
+}
+
 // Type assert containerReference implements ExecutionsEnvironment
 var _ ExecutionsEnvironment = &containerReference{}