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/.github/workflows/local-reusable-workflow.yml

@@ -0,0 +1,34 @@
+name: local-reusable-workflow
+on:
+  workflow_call:
+    inputs:
+      string_required:
+        required: true
+        type: string
+      bool_required:
+        required: true
+        type: boolean
+      number_required:
+        required: true
+        type: number
+    secrets:
+      secret:
+        required: true
+    outputs:
+      output:
+        value: ${{ jobs.reusable.outputs.output }}
+
+jobs:
+  reusable:
+    runs-on: ubuntu-latest
+    outputs:
+      output: ${{ steps.gen.outputs.output }}
+    steps:
+      - name: check inputs and secret arrived
+        run: |
+          [ "${{ inputs.string_required }}" = "string" ]
+          [ "${{ inputs.bool_required }}" = "true" ]
+          [ "${{ inputs.number_required }}" = "1" ]
+          [ "${{ secrets.secret }}" = "keep_it_private" ]
+      - id: gen
+        run: echo "output=${{ inputs.string_required }}" >> $GITHUB_OUTPUT