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>

.gitea/workflows/test.yml

@@ -9,14 +9,36 @@ jobs:
   lint:
     name: check and test
     runs-on: ubuntu-latest
+    env:
+      # The runner image ships a stale docker.io login; point docker at an empty config so
+      # image pulls go straight to anonymous instead of attempting (and failing) that auth
+      # first. The path must be a literal: the `runner` context is unavailable in job-level
+      # env, so `${{ runner.temp }}` would resolve to empty and config.Dir() would fall back
+      # to ~/.docker with the stale credentials.
+      DOCKER_CONFIG: /tmp/docker-noauth
     steps:
       - uses: actions/checkout@v6
       - uses: actions/setup-go@v6
         with:
           go-version-file: 'go.mod'
+      - name: prepare anonymous docker config
+        run: mkdir -p "$DOCKER_CONFIG" && echo '{}' > "$DOCKER_CONFIG/config.json"
+      # Pre-pull act/runner's two largest base images so a slow pull can't dominate `make test`;
+      # the rest (alpine/ubuntu) pull on demand, absorbed by the make-test -timeout. The host
+      # daemon retains them between runs, so this is usually a fast manifest re-check.
+      - name: pre-pull test images
+        run: |
+          for img in node:24-bookworm-slim nginx:alpine; do
+            for try in 1 2 3; do docker pull "$img" && break || sleep 5; done
+          done
       - name: lint
         run: make lint
       - name: build
         run: make build
       - name: test
-        run: make test
+        run: make test
+      # Build the dind image and run the daemon-facing tests against the docker version it
+      # ships, catching daemon-level regressions (e.g. gitea/runner#981) before release. Runs
+      # after `make test` so the images it needs are already present on the host daemon.
+      - name: test against dind image
+        run: make test-dind