Tweak Compose file so that it works out of the box

Fix umap image reference to match the one pushed on Docker Hub.

Also change port to an env var with a default value,
so that it works out of the box but can also be easily
changed to another port if needed.

docker-compose.yml

@@ -1,5 +1,7 @@
 version: '3'
+
 services:
+
   db:
     image: postgis/postgis:14-3.3-alpine
     environment:
@@ -8,9 +10,9 @@ services:
       - db:/var/lib/postgresql/data
 
   app:
-    image: umap:1.3.0
+    image: umap/umap:1.3.2
     ports:
-      - "8001:8000"
+      - "${PORT-8000}:8000"
     environment:
       - DATABASE_URL=postgis://postgres@db/postgres
       - SECRET_KEY=some-long-and-weirdly-unrandom-secret-key