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 compatability.

Fixes #2074

docker/assets/update-permissions

@@ -45,6 +45,6 @@ if id -u gitlab-redis; then
 fi
 # Fix registry storage
-if id -u registry; then
+if id -u registry && [ -e /var/opt/gitlab/gitlab-rails/shared/registry ] ; then
     chown -R registry:registry /var/opt/gitlab/gitlab-rails/shared/registry
 fi