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/runner/run_context_test.go

@@ -5,6 +5,7 @@
 package runner
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"os"
@@ -12,6 +13,7 @@ import (
 	"strings"
 	"testing"
 
+	"gitea.com/gitea/runner/act/common"
 	"gitea.com/gitea/runner/act/exprparser"
 	"gitea.com/gitea/runner/act/model"
 
@@ -635,3 +637,25 @@ func TestCreateContainerNameBoundedForLongMatrixInput(t *testing.T) {
 	assert.LessOrEqual(t, len(name+"-network"), 255)
 	assert.LessOrEqual(t, len(name+"-job1234567890"), 255)
 }
+
+func TestPrintStartJobContainerGroupGolden(t *testing.T) {
+	buf := &bytes.Buffer{}
+	logger := log.New()
+	logger.SetOutput(buf)
+	logger.SetLevel(log.InfoLevel)
+	logger.SetFormatter(&jobLogFormatter{color: cyan})
+	entry := logger.WithFields(log.Fields{"job": "j1"})
+	ctx := common.WithLogger(context.Background(), entry)
+
+	printStartJobContainerGroup(ctx, "node:20", "GITEA-WORKFLOW-build-JOB-test", "gitea-runner-network")()
+
+	want := strings.Join([]string{
+		"[j1]   | ::group::Starting job container",
+		"[j1]   | image: node:20",
+		"[j1]   | name: GITEA-WORKFLOW-build-JOB-test",
+		"[j1]   | network: gitea-runner-network",
+		"[j1]   | ::endgroup::",
+		"",
+	}, "\n")
+	assert.Equal(t, want, buf.String())
+}