dev: add reference docker-compose for getting postgres up

Required environment variable for the test: `DPP_PG_URL`.

    export DPP_PG_URL="postgres://postgres:localhost@127.0.0.1:5432/dino-park-packs-test-local"

Stash this in .envrc or similar.

Jira: IAM-1908

Author: bheesham

dev/docker-compose.yml

@@ -0,0 +1,10 @@
+# This is used for local development only.
+
+services:
+  postgres:
+    image: postgres:18.3
+    ports:
+    - 127.0.0.1:5432:5432
+    environment:
+      POSTGRES_DB: dino-park-packs-test-local
+      POSTGRES_PASSWORD: localhost