fix: stop truncating existing files in common.TouchFile

`common.TouchFile` opened files with `O_TRUNC`, so the call from `common.SshTask.Execute` against `~/.ssh/known_hosts` emptied the dokku user's previously trusted host keys before `goph.DefaultKnownHosts()` read them. Running `scheduler-k3s:cluster:add` without `--insecure-allow-unknown-hosts` then failed with `knownhosts: key is unknown` and left a zero-byte `known_hosts`. The function now matches `touch(1)` semantics: create the file if missing, otherwise leave its contents intact.

plugins/common/io.go

@@ -212,10 +212,12 @@ func TouchDir(filename string) error {
 	return os.MkdirAll(filename, mode)
 }
 
-// TouchFile creates an empty file at the specified path
+// TouchFile ensures a file exists at the specified path without truncating
+// its contents. It matches the semantics of unix touch(1): create the file if
+// it does not exist, otherwise leave the existing contents intact.
 func TouchFile(filename string) error {
 	mode := os.FileMode(0600)
-	file, err := os.OpenFile(filename, os.O_RDWR|os.O_CREATE|os.O_TRUNC, mode)
+	file, err := os.OpenFile(filename, os.O_WRONLY|os.O_CREATE, mode)
 	if err != nil {
 		return fmt.Errorf("Error opening file %v for creation: %v", filename, err)
 	}

plugins/common/io_test.go

@@ -1,6 +1,9 @@
 package common
 
 import (
+	"os"
+	"os/user"
+	"path/filepath"
 	"testing"
 
 	. "github.com/onsi/gomega"
@@ -31,3 +34,47 @@ func TestCommonReadFirstLine(t *testing.T) {
 	Expect(line).To(Equal(testEnvLine))
 	teardownTestApp()
 }
+
+// setupTouchFileTest pins DOKKU_SYSTEM_USER/GROUP to the running test user so
+// SetPermissions' chown succeeds for non-root test runs.
+func setupTouchFileTest(t *testing.T) {
+	t.Helper()
+	u, err := user.Current()
+	Expect(err).NotTo(HaveOccurred())
+	g, err := user.LookupGroupId(u.Gid)
+	Expect(err).NotTo(HaveOccurred())
+	t.Setenv("DOKKU_SYSTEM_USER", u.Username)
+	t.Setenv("DOKKU_SYSTEM_GROUP", g.Name)
+}
+
+func TestCommonTouchFileCreatesMissingFile(t *testing.T) {
+	RegisterTestingT(t)
+	setupTouchFileTest(t)
+
+	path := filepath.Join(t.TempDir(), "new-file")
+	Expect(FileExists(path)).To(BeFalse())
+	Expect(TouchFile(path)).To(Succeed())
+
+	info, err := os.Stat(path)
+	Expect(err).NotTo(HaveOccurred())
+	Expect(info.Size()).To(BeNumerically("==", 0))
+	Expect(info.Mode().Perm()).To(Equal(os.FileMode(0600)))
+}
+
+// TestCommonTouchFilePreservesExistingContents is a regression test for #8722:
+// scheduler-k3s:cluster:add used to zero out the dokku user's ~/.ssh/known_hosts
+// because TouchFile opened the file with O_TRUNC.
+func TestCommonTouchFilePreservesExistingContents(t *testing.T) {
+	RegisterTestingT(t)
+	setupTouchFileTest(t)
+
+	path := filepath.Join(t.TempDir(), "existing-file")
+	contents := []byte("host ssh-rsa AAAA...\n")
+	Expect(os.WriteFile(path, contents, 0600)).To(Succeed())
+
+	Expect(TouchFile(path)).To(Succeed())
+
+	got, err := os.ReadFile(path)
+	Expect(err).NotTo(HaveOccurred())
+	Expect(got).To(Equal(contents))
+}