feat: make pseudo-TTY allocation opt-in (#961)

Fixes #956.

Pseudo-TTY allocation is now an explicit, runner-wide opt-in via `runner.allocate_pty`, applied to both host and docker backends. Default is off, matching GitHub `actions/runner`.

```yaml
runner:
  allocate_pty: false  # default
```

**Before:** the host backend hardcoded `if true /* allocate Terminal */` and the docker backend used `term.IsTerminal(os.Stdout.Fd())`. As a result, `docker build` (and other TTY-aware tools) saw a TTY and emitted cursor-control redraw frames that flooded captured logs with thousands of duplicate-looking progress lines — only on host-mode runners in production, and on docker-mode runners when the daemon happened to be launched from a shell rather than a service.

**After:** both backends consult `Config.AllocatePTY`. The `term.IsTerminal` heuristic is gone, so behavior no longer depends on whether the daemon has a controlling terminal.

**Reproduction:** running `docker build` through `HostEnvironment.Exec` with output captured to a buffer:

| | Before (`if true`) | After (`AllocatePTY=false`) |
|---|---:|---:|
| bytes captured | 18,167 | 1,048 |
| ANSI CSI sequences | 556 | 0 |
| cursor-up `\e[1A` | 181 | 0 |

**Side fix:** `ptyWriter.AutoStop` is now `atomic.Bool`. The field is written from the exec goroutine after `cmd.Wait()` and read from the `copyPtyOutput` goroutine via `ptyWriter.Write`; existing tests never tripped the race detector because their commands produced no output before exit. The new host-mode test does.

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

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Nicolas <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/961
Reviewed-by: Nicolas <bircni@icloud.com>
Co-authored-by: silverwind <2021+silverwind@noreply.gitea.com>
Co-committed-by: silverwind <2021+silverwind@noreply.gitea.com>

act/container/host_environment.go

@@ -19,6 +19,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"gitea.com/gitea/runner/act/common"
@@ -42,6 +43,7 @@ type HostEnvironment struct {
 	ActPath     string
 	CleanUp     func()
 	StdOut      io.Writer
+	AllocatePTY bool // allocate a pseudo-TTY for each step's process
 
 	mu          sync.Mutex
 	runningPIDs map[int]struct{}
@@ -200,12 +202,12 @@ func (e *HostEnvironment) Start(_ bool) common.Executor {
 
 type ptyWriter struct {
 	Out       io.Writer
-	AutoStop  bool
+	AutoStop  atomic.Bool
 	dirtyLine bool
 }
 
 func (w *ptyWriter) Write(buf []byte) (int, error) {
-	if w.AutoStop && len(buf) > 0 && buf[len(buf)-1] == 4 {
+	if w.AutoStop.Load() && len(buf) > 0 && buf[len(buf)-1] == 4 {
 		n, err := w.Out.Write(buf[:len(buf)-1])
 		if err != nil {
 			return n, err
@@ -335,21 +337,20 @@ func (e *HostEnvironment) exec(ctx context.Context, command []string, cmdline st
 			tty.Close()
 		}
 	}()
-	if true /* allocate Terminal */ {
+	if e.AllocatePTY {
 		var err error
 		ppty, tty, err = setupPty(cmd, cmdline)
 		if err != nil {
 			common.Logger(ctx).Debugf("Failed to setup Pty %v\n", err.Error())
 		}
 	}
-	writer := &ptyWriter{Out: e.StdOut}
-	logctx, finishLog := context.WithCancel(context.Background())
+	var writer *ptyWriter
+	var logctx context.Context
 	if ppty != nil {
+		writer = &ptyWriter{Out: e.StdOut}
+		var finishLog context.CancelFunc
+		logctx, finishLog = context.WithCancel(context.Background())
 		go copyPtyOutput(writer, ppty, finishLog)
-	} else {
-		finishLog()
-	}
-	if ppty != nil {
 		go writeKeepAlive(ppty)
 	}
 	// Split Start/Wait so the PID can be registered before the process can exit;
@@ -379,14 +380,11 @@ func (e *HostEnvironment) exec(ctx context.Context, command []string, cmdline st
 		return err
 	}
 	if tty != nil {
-		writer.AutoStop = true
+		writer.AutoStop.Store(true)
 		if _, err := tty.WriteString("\x04"); err != nil {
 			common.Logger(ctx).Debug("Failed to write EOT")
 		}
-	}
-	<-logctx.Done()
-
-	if ppty != nil {
+		<-logctx.Done()
 		ppty.Close()
 		ppty = nil
 	}