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/runner/helpers_test.go

@@ -0,0 +1,66 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package runner
+
+import (
+	"context"
+	"net"
+	"os/exec"
+	"runtime"
+	"testing"
+	"time"
+
+	"gitea.com/gitea/runner/act/container"
+
+	mobyclient "github.com/moby/moby/client"
+)
+
+// requireLinuxDocker skips on non-Linux hosts. Some integration workflows need Docker features
+// that only a Linux daemon provides (host networking, host /proc bind mounts); Docker Desktop
+// on macOS/Windows does not, so those tests can only run on Linux.
+func requireLinuxDocker(t *testing.T) {
+	t.Helper()
+	if runtime.GOOS != "linux" {
+		t.Skip("skipping: requires a Linux Docker host")
+	}
+}
+
+// requireDocker skips the test unless a reachable docker daemon is available.
+// GetDockerClient succeeds even without a running daemon (its ping is best-effort),
+// so the daemon has to be pinged explicitly here to decide whether to skip.
+func requireDocker(t *testing.T) {
+	t.Helper()
+	ctx := context.Background()
+	cli, err := container.GetDockerClient(ctx)
+	if err != nil {
+		t.Skipf("skipping: docker client unavailable: %v", err)
+	}
+	defer cli.Close()
+	if _, err := cli.Ping(ctx, mobyclient.PingOptions{}); err != nil {
+		t.Skipf("skipping: docker daemon unreachable: %v", err)
+	}
+}
+
+// requireNetwork skips the test unless github.com is reachable. A few tests exercise behaviour
+// that inherently needs the network (force-pulling an image, resolving a remote short-sha ref);
+// gating lets the rest of the suite run offline without these failing.
+func requireNetwork(t *testing.T) {
+	t.Helper()
+	conn, err := net.DialTimeout("tcp", "github.com:443", 3*time.Second)
+	if err != nil {
+		t.Skipf("skipping: network unavailable: %v", err)
+	}
+	_ = conn.Close()
+}
+
+// requireHostTools skips the test unless every named executable is on PATH. Used by the
+// self-hosted (host environment) suite, which runs steps directly on the host.
+func requireHostTools(t *testing.T, tools ...string) {
+	t.Helper()
+	for _, tool := range tools {
+		if _, err := exec.LookPath(tool); err != nil {
+			t.Skipf("skipping: required host tool %q not found: %v", tool, err)
+		}
+	}
+}