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/reusable_workflow.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"net/url"
 	"path"
+	"path/filepath"
 	"regexp"
 	"strings"
 
@@ -27,7 +28,9 @@ func newLocalReusableWorkflowExecutor(rc *RunContext) common.Executor {
 		workflowDir = strings.TrimPrefix(workflowDir, "./")
 
 		return common.NewPipelineExecutor(
-			newReusableWorkflowExecutor(rc, workflowDir, fileName),
+			// resolve the local workflow against the workspace root, not the process
+			// working directory, so it is found regardless of where the runner is invoked
+			newReusableWorkflowExecutor(rc, filepath.Join(rc.Config.Workdir, workflowDir), fileName),
 		)
 	}
 
@@ -284,7 +287,11 @@ func setReusedWorkflowCallerResult(rc *RunContext, runner Runner) common.Executo
 			if rc.caller != nil {
 				rc.caller.setReusedWorkflowJobResult(rc.JobName, reusedWorkflowJobResult)
 			} else {
+				// Serialize this shared Job.Result write against the other matrix combos
+				// and setJobResult (same lockJob key).
+				unlock := lockJob(rc.Run.Job())
 				rc.result(reusedWorkflowJobResult)
+				unlock()
 				logger.WithField("jobResult", reusedWorkflowJobResult).Infof("Job %s", reusedWorkflowJobResultMessage)
 			}
 		}