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/step_docker.go

@@ -138,7 +138,7 @@ func (sd *stepDocker) newStepContainer(ctx context.Context, image string, cmd, e
 		UsernsMode:   rc.Config.UsernsMode,
 		Platform:     rc.Config.ContainerArchitecture,
 		AutoRemove:   rc.Config.AutoRemove,
-		ValidVolumes: rc.Config.ValidVolumes,
+		ValidVolumes: rc.validVolumes(),
 		AllocatePTY:  rc.Config.AllocatePTY,
 	})
 	return stepContainer