The command was hijacking auth for all users, including `gitea`, which
broke `git push` over SSH — `gitea`'s `authorized_keys` (with the
`gitea serv` command restriction) was being bypassed, and sshd would
try to exec the raw `git-receive-pack` instead.
Pass `%u` to the command and short-circuit unless the requested user
is `alex`, so other users fall back to their own `~/.ssh/authorized_keys`.
Alexander Heldt