feat: restructure PostgreSQL setup with base variant

postgres-base/compose.yml

@@ -0,0 +1,16 @@
+services:
+  postgres:
+    # run `just build postgres` first
+    image: db-postgres:latest
+    restart: unless-stopped
+    environment:
+      POSTGRES_USER: ${POSTGRES_USER:-postgres}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-postgres}
+      POSTGRES_DB: ${POSTGRES_DB:-postgres}
+    ports:
+      - "${POSTGRES_PORT:-5432}:5432"
+    volumes:
+      - postgres-data:/var/lib/postgresql
+
+volumes:
+  postgres-data: