Docker cleanups

- Put docker-compose.yml into version control, no need to make this an example
  file and complicate things.
- Start the docker dev-env osem on port 3000. Just as our documentation says
  and how we do it in vagrant.
- hardcode CONTAINER_USERID so you don't have to have an
  docker-compose.override.yml if your UID is 1000.
- Rename web service/container to osem for clarity in the logs
- Keep assets also in a volume in production
- warn a bit more prominently about docker-compose.override.yml

docker-compose.yml.dev-example

@@ -1,17 +0,0 @@
-version: "2"
-
-services:
-  database:
-    image: postgres
-    environment:
-      PGDATA: /var/lib/postgresql/data/pgdata
-  web:
-    build: .
-    # env_file: dotenv.example
-    command: /osem/bin/osem-init.sh 
-    depends_on:
-      - database
-    ports:
-      - 5000:5000
-    volumes:
-      - .:/osem