docker/root/usr/bin/entrypoint already allows for the specification
of USER, USER_UID, USER_GID. But since AllowedUsers is hardcoded in
sshd_config, one cannot log in as a user different ftom git.
This change substitutes ${USER} for git in the sshd_config template.

Signed-off-by: Jeronimo Pellegrini <j_p@aleph0.info>

* migrate database if app.ini found

* replacing hard-coded user id by env variable

* Update per @zeripath's feedback

Should fix #5623

* If using a different $USER then rename git user

* Chown based on $USER env

* Target only one part of passwd

* su-exec based on $USER

not a hardcoded value

I have restructured the docker build process entirely, the binary gets
built outside of the docker build command, now we are managing all
dependencies with real Alpine packages and I have dropped features like
socat or the cron daemon.

Signed-off-by: Thomas Boerger <tboerger@suse.de>