De-duplicate environment variable handling in docker-compose files

Currently, the docker-compose files all rely on docker-compose reading a `.env` file, and then we manually add each environment variable to the `environment` field in `docker-compose.yml`. For SvelteKit, we should be able to write variables to `.env.local` and mount that into the container instead, and this will make things less confusing if we need to add variables in the future as they'll only need adding in the one place.