feat: remove emojis from runner logging, add Starting job container group (#940)

Aligns runner log output more closely with `actions/runner`:

- Strip the whale, rocket, cloud, construction, chequered-flag, and exclamation-mark glyphs from log lines and drop the now-unused `logPrefix` constant.
- Reword `no outputs used step '%s'` → `No outputs registered for step '%s'` (the original was ungrammatical and inaccurate — it fires when `set-output` references an unknown step ID).
- Wrap the docker pull/network/create/start phase of job container startup in a `::group::Starting job container` / `::endgroup::` collapsible section, mirroring `actions/runner`. Since act drives Docker through the SDK rather than the CLI, we can't echo `##[command]/usr/bin/docker create ...` lines verbatim — instead the helper emits a summary inside the group:

  ```
  ::group::Starting job container
  image: <image>
  name: <container-name>
  network: <network-name>
  ::endgroup::
  ```

- Extracted the emit into a `printStartJobContainerGroup` helper (parallel to `printRunActionHeader` in `step_run.go`) and added a golden-style test `TestPrintStartJobContainerGroupGolden`.
- Drive-by: replace two remaining literal `"raw_output"` strings in `run_context.go` with the existing `rawOutputField` constant.

Closes #935

---
This PR was written with the help of Claude Opus 4.7

Reviewed-on: https://gitea.com/gitea/runner/pulls/940
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

act/container/docker_build.go

@@ -25,9 +25,9 @@ func NewDockerBuildExecutor(input NewDockerBuildExecutorInput) common.Executor {
 	return func(ctx context.Context) error {
 		logger := common.Logger(ctx)
 		if input.Platform != "" {
-			logger.Infof("%sdocker build -t %s --platform %s %s", logPrefix, input.ImageTag, input.Platform, input.ContextDir)
+			logger.Infof("docker build -t %s --platform %s %s", input.ImageTag, input.Platform, input.ContextDir)
 		} else {
-			logger.Infof("%sdocker build -t %s %s", logPrefix, input.ImageTag, input.ContextDir)
+			logger.Infof("docker build -t %s %s", input.ImageTag, input.ContextDir)
 		}
 		if common.Dryrun(ctx) {
 			return nil