fix(docker): don't pass server-only vars when building client bundle

Learning some more about Dockerfiles, I realise that these env variables we're passing through to the Dockerfiles as ARGs are used for building the client app bundle. We should not be sending through server-only variables, and especially not secrets -- otherwise they can be retrieved from within the client bundle. Essentially, the variables we're passing to the client in docker-compose.yml are the same variables we should be passing through as args to the Dockerfiles.