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

@@ -0,0 +1,19 @@
+version: "2"
+
+services:
+  database:
+    image: postgres
+    environment:
+      PGDATA: /var/lib/postgresql/data/pgdata
+  osem:
+    build:
+      context: .
+      args:
+        CONTAINER_USERID: 1000
+    command: /osem/bin/osem-init.sh 
+    depends_on:
+      - database
+    ports:
+      - 3000:3000
+    volumes:
+      - .:/osem