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/testdata/issue-598/spelling.yml

@@ -1,31 +1,21 @@
 name: issue-598
 on: push
-  
+
 
 jobs:
   my_first_job:
-    
+
     runs-on: ubuntu-latest
     steps:
       - name: My first false step
         if: "endsWith('Hello world', 'o1')"
-        uses: actions/hello-world-javascript-action@main
-        with:
-          who-to-greet: 'Mona the Octocat'
+        run: exit 1
       - name: My first true step
         if: "!endsWith('Hello world', 'od')"
-        uses: actions/hello-world-javascript-action@main
-        with:
-          who-to-greet: "Renst the Octocat"
+        run: echo "Renst the Octocat"
       - name: My second false step
         if: "endsWith('Hello world', 'o2')"
-        uses: actions/hello-world-javascript-action@main
-        with:
-          who-to-greet: 'Act the Octocat'
+        run: exit 1
       - name: My third false step
         if: "endsWith('Hello world', 'o2')"
-        uses: actions/hello-world-javascript-action@main
-        with:
-          who-to-greet: 'Git the Octocat'
-           
-           
+        run: exit 1