Fix host cleanup, volume allowlist, cache upload, and action host edge cases (#970)

## Summary
- prevent host-mode execution from deleting caller-owned workdirs
- harden `valid_volumes` checks against `..` and symlink escapes
- return immediately after artifact cache upload write failures
- default implicit remote action clone hosts to `GitHubInstance`/`github.com`

Authored with assistance from OpenAI Codex GPT-5.

---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/970
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>

act/container/docker_run_test.go

@@ -11,6 +11,8 @@ import (
 	"errors"
 	"io"
 	"net"
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 	"time"
@@ -375,3 +377,40 @@ func TestCheckVolumes(t *testing.T) {
 		})
 	}
 }
+
+func TestCheckVolumesRejectsEscapingHostPaths(t *testing.T) {
+	logger, _ := test.NewNullLogger()
+	ctx := common.WithLogger(context.Background(), logger)
+
+	base := t.TempDir()
+	allowed := filepath.Join(base, "allowed")
+	denied := filepath.Join(base, "denied")
+	require.NoError(t, os.MkdirAll(allowed, 0o700))
+	require.NoError(t, os.MkdirAll(denied, 0o700))
+
+	cr := &containerReference{
+		input: &NewContainerInput{
+			ValidVolumes: []string{filepath.Join(allowed, "**")},
+		},
+	}
+
+	escapingPath := allowed + string(filepath.Separator) + ".." + string(filepath.Separator) + "denied"
+	_, hostConf := cr.sanitizeConfig(ctx, &container.Config{}, &container.HostConfig{
+		Binds: []string{escapingPath + ":/mnt"},
+	})
+	assert.Empty(t, hostConf.Binds)
+
+	linkPath := filepath.Join(allowed, "link")
+	if err := os.Symlink(denied, linkPath); err != nil {
+		t.Skipf("cannot create symlink: %v", err)
+	}
+	_, hostConf = cr.sanitizeConfig(ctx, &container.Config{}, &container.HostConfig{
+		Binds: []string{linkPath + ":/mnt"},
+	})
+	assert.Empty(t, hostConf.Binds)
+
+	_, hostConf = cr.sanitizeConfig(ctx, &container.Config{}, &container.HostConfig{
+		Binds: []string{filepath.Join(linkPath, "missing") + ":/mnt"},
+	})
+	assert.Empty(t, hostConf.Binds)
+}