Add -u git to docs when using docker exec with root installation (#29314) (#30259)

Backport #29314 by @scribblemaniac

This fixes a minor issue in the documentation for SSH Container
Passthrough for non-rootless installs. The non-rootless Dockerfile and
docker-compose do not set `USER`/`user` instructions so `docker exec`
will run as root by default. While running as root, gitea commands will
refuse to execute, breaking these approaches. For containers built with
the rootless instructions, `docker exec` will run as git by default so
this is not necessary in that case.

This issue was already discussed in #19065, but it does not appear this
part of the issue was ever added to the documentation.

Co-authored-by: scribblemaniac <scribblemaniac@users.noreply.github.com>

docs/content/installation/with-docker.en-us.md

@@ -545,7 +545,7 @@ In this option, the idea is that the host SSH uses an `AuthorizedKeysCommand` in
   ```bash
   cat <<"EOF" | sudo tee /home/git/docker-shell
   #!/bin/sh
-  /usr/bin/docker exec -i --env SSH_ORIGINAL_COMMAND="$SSH_ORIGINAL_COMMAND" gitea sh "$@"
+  /usr/bin/docker exec -i -u git --env SSH_ORIGINAL_COMMAND="$SSH_ORIGINAL_COMMAND" gitea sh "$@"
   EOF
   sudo chmod +x /home/git/docker-shell
   sudo usermod -s /home/git/docker-shell git
@@ -560,7 +560,7 @@ Add the following block to `/etc/ssh/sshd_config`, on the host:
 ```bash
 Match User git
   AuthorizedKeysCommandUser git
-  AuthorizedKeysCommand /usr/bin/docker exec -i gitea /usr/local/bin/gitea keys -c /data/gitea/conf/app.ini -e git -u %u -t %t -k %k
+  AuthorizedKeysCommand /usr/bin/docker exec -i -u git gitea /usr/local/bin/gitea keys -c /data/gitea/conf/app.ini -e git -u %u -t %t -k %k
 ```
 
 (From 1.16.0 you will not need to set the `-c /data/gitea/conf/app.ini` option.)