Add Docker-Compose setup with configurable local port via .env.

Document container startup and local port configuration in README, including a .env example for quick setup.

Made-with: Cursor

docker-compose.yml

@@ -0,0 +1,8 @@
+services:
+  datecalc:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    ports:
+      - "${LOCAL_PORT:-5000}:5000"
+    restart: unless-stopped