Docker: update-permissions, only touch registry if exists

Add an exists check to the update-permissions script, so that it only attempts to set the permissions on the registry (/var/opt/gitlab/gitlab-rails/shared/registry) if it actually exists. This removes the spurious "fail" on setting permissions on things that don't exist, and allows for better scripting compatibility.

Set the /var/opt/gitlab/redis permissions to gitlab-redis:git if the git user exists. This brings the settings in-line with what is expected in the environment for the git user to be able to access the Redis instance.

Closes #2074 (closed)