Correct permissions for `.ssh` and `authorized_keys` (#25721) (#25731)

Correct permissions for `.ssh` and `authorized_keys` (#25721) (#25731)

Backport #25721 by @wolfogre

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: Jason Song <i@wolfogre.com>