fix: apply a job container without image to the runs-on image (#1252)

Fixes https://gitea.com/gitea/runner/issues/1251, a regression from https://gitea.com/gitea/runner/pulls/1233.

A job `container:` without an `image` key configures the runs-on image again, as in 3.x, so its `volumes`, `options`, `credentials` and `env` apply. An image that evaluates to empty still skips the container, as on GitHub.

---------

Co-authored-by: bircni <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1252
Reviewed-by: Lunny Xiao <[email protected]>
Reviewed-by: bircni <[email protected]>
Co-authored-by: silverwind <[email protected]>

e2e/compatibility_test.go

@@ -69,6 +69,7 @@ func testPayloads(t *testing.T) {
 		"cell-a-2",
 		"cell-b-1",
 		"cell-b-2",
+		"imageless env=imageless-env-value host=imageless-host",
 	} {
 		if !strings.Contains(logs, want) {
 			t.Errorf("stored logs are missing %q", want)