fix: use version 3.8 of the Docker Compose specification

See also the Compose and Docker compatibility matrix [1].

While at it, reorder top-level configuration elements and
separate each by a blank line to enhance readability.

[1] https://docs.docker.com/compose/compose-file/compose-file-v3/#compose-and-docker-compatibility-matrix

docker-compose/postgres/docker-compose.yaml

@@ -1,10 +1,6 @@
 ---
-# (Optional) when using custom network
+version: "3.8"
-# networks:
+
-#   yournetwork:
-#     external: true
-volumes:
-  postgres-data:
 services:
   postgres:
     # (Recommended) replace "latest" with specific version
@@ -20,5 +16,13 @@ services:
     # networks:
     #   - yournetwork
     volumes:
-      - postgres-data:/var/lib/postgresql/data
+      - postgres_data:/var/lib/postgresql/data
     restart: unless-stopped
+
+# (Optional) when using custom network
+# networks:
+#   yournetwork:
+#     external: true
+
+volumes:
+  postgres_data: