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/services-with-container/push.yml

@@ -5,12 +5,11 @@ jobs:
     runs-on: ubuntu-latest
     # https://docs.github.com/en/actions/using-containerized-services/about-service-containers#running-jobs-in-a-container
     container:
-      image: "ubuntu:latest"
+      image: "node:24-bookworm-slim"
     services:
       nginx:
-        image: "nginx:latest"
-        ports:
-          - "8080:80"
+        image: "nginx:alpine"
     steps:
       - run: apt-get -qq update && apt-get -yqq install --no-install-recommends curl
+      # reach the service over the shared job network by its alias, no host port needed
       - run: curl -v http://nginx:80