Add -u git to docs when using docker exec with root installation (#29314) (#30264)

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>