Correct permissions for `.ssh` and `authorized_keys` (#25721)

Set the correct permissions on the .ssh directory and authorized_keys
file, or sshd will refuse to use them and lead to clone/push/pull
failures.

It could happen when users have copied their data to a new volume and
changed the file permission by accident, and it would be very hard to
troubleshoot unless users know how to check the logs of sshd which is
started by s6.

Co-authored-by: Giteabot <teabot@gitea.io>

docker/root/etc/s6/gitea/setup

@@ -2,7 +2,15 @@
 
 if [ ! -d /data/git/.ssh ]; then
     mkdir -p /data/git/.ssh
-    chmod 700 /data/git/.ssh
+fi
+
+# Set the correct permissions on the .ssh directory and authorized_keys file,
+# or sshd will refuse to use them and lead to clone/push/pull failures.
+# It could happen when users have copied their data to a new volume and changed the file permission by accident,
+# and it would be very hard to troubleshoot unless users know how to check the logs of sshd which is started by s6.
+chmod 700 /data/git/.ssh
+if [ -f /data/git/.ssh/authorized_keys ]; then
+    chmod 600 /data/git/.ssh/authorized_keys
 fi
 
 if [ ! -f /data/git/.ssh/environment ]; then