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_action_remote.go

@@ -291,7 +291,9 @@ type remoteAction struct {
 
 func (ra *remoteAction) CloneURL(u string) string {
 	if ra.URL == "" {
-		if !strings.HasPrefix(u, "http://") && !strings.HasPrefix(u, "https://") {
+		// keep an absolute local path as-is (used by tests to resolve actions from a local
+		// repo); only bare host names get the https:// scheme prepended
+		if !strings.HasPrefix(u, "http://") && !strings.HasPrefix(u, "https://") && !filepath.IsAbs(u) {
 			u = "https://" + u
 		}
 	} else {