Update .gitlab-ci.yml to use new portus registry

1 job for master in 26 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
failed #339
build

00:00:26

 
Name Stage Failure
failed
build Build
Status: Image is up to date for wordpress:php7.3-fpm
docker.io/library/wordpress:php7.3-fpm
$ docker pull $REGISTRY_IMAGE:latest || true
Error response from daemon: manifest for registry.dark.kow.is/kowis/wordpress-container:latest not found: manifest unknown: manifest unknown
$ cd wordpress-container
$ docker build --cache-from $REGISTRY_IMAGE:latest --tag $REGSITRY_IMAGE:latest .
invalid argument ":latest" for "-t, --tag" flag: invalid reference format
See 'docker build --help'.
ERROR: Job failed: exit code 125