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/container/docker_auth.go

@@ -13,7 +13,6 @@ import (
 
 	"github.com/distribution/reference"
 	"github.com/docker/cli/cli/config"
-	"github.com/docker/cli/cli/config/credentials"
 	"github.com/moby/moby/api/types/registry"
 )
 
@@ -26,10 +25,6 @@ func LoadDockerAuthConfig(ctx context.Context, image string) (registry.AuthConfi
 		logger.Warnf("Could not load docker config: %v", err)
 		return registry.AuthConfig{}, err
 	}
-	if !cfg.ContainsAuth() {
-		cfg.CredentialsStore = credentials.DetectDefaultStore(cfg.CredentialsStore)
-	}
-
 	registryKey := registryAuthConfigKey("docker.io")
 	if image != "" {
 		if registryRef, refErr := reference.ParseNormalizedNamed(image); refErr != nil {
@@ -55,10 +50,6 @@ func LoadDockerAuthConfigs(ctx context.Context) map[string]registry.AuthConfig {
 		logger.Warnf("Could not load docker config: %v", err)
 		return nil
 	}
-	if !cfg.ContainsAuth() {
-		cfg.CredentialsStore = credentials.DetectDefaultStore(cfg.CredentialsStore)
-	}
-
 	creds, err := cfg.GetAllCredentials()
 	if err != nil {
 		logger.Warnf("Could not get docker auth configs: %v", err)