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>