fix: restore global docker config dir and socket env in tests (#1004)

`TestGetImagePullOptions` left docker/cli's process-global config dir pointed at `testdata/docker-pull-options` (which ships dummy `username:password` creds) via `config.SetDir`, without restoring it. Because that override is process-global, every later docker-gated test in the package then pulled with those creds — `TestDockerCopyToSymlinkPath`'s `alpine:latest` pull failed with `incorrect username or password` and broke CI. The workflow's `DOCKER_CONFIG` override can't mask this, since `SetDir` wins in-process.

Restore `config.Dir()` with `t.Cleanup`, and isolate the socket tests' leaks of the exported `CommonSocketLocations` and `DOCKER_HOST` behind an `isolateSocketEnv` helper.

Refs https://gitea.com/gitea/gitea.com/issues/83

---
This PR was written with the help of Claude Opus 4.8

Reviewed-on: https://gitea.com/gitea/runner/pulls/1004
Reviewed-by: Zettat123 <[email protected]>
Reviewed-by: Lunny Xiao <[email protected]>
Co-authored-by: silverwind <[email protected]>
Co-committed-by: silverwind <[email protected]>

act/container/docker_pull_test.go

@@ -40,6 +40,9 @@ func TestCleanImage(t *testing.T) {
 func TestGetImagePullOptions(t *testing.T) {
 	ctx := context.Background()
 
+	orig := config.Dir()
+	t.Cleanup(func() { config.SetDir(orig) })
+
 	config.SetDir("/non-existent/docker")
 
 	options, err := getImagePullOptions(ctx, NewDockerPullExecutorInput{})