feat: make host port configurable via APP_PORT env var

Allow users to customize the host-side port mapping while keeping the
container's internal port fixed at 3000. The default remains 3000 when
APP_PORT is unset.

- Add APP_PORT to .env.example with documentation
- Update docker-compose.yml to use ${APP_PORT:-3000}:3000
- Update README with new env var and usage notes

.env.example

@@ -12,3 +12,7 @@ AUTH_SECRET="replace-with-a-generated-secret"
 POSTGRES_USER=organize
 POSTGRES_PASSWORD=organize
 POSTGRES_DB=organize
+
+# Host port the "app" service is published on (e.g. http://localhost:3000).
+# The container always listens on 3000 internally regardless of this value.
+APP_PORT=3000

README.md

@@ -69,10 +69,11 @@ the Compose plugin.
    migrations automatically on every boot before serving traffic. No
    separate migration or setup step is needed.
 
-4. **Open the app** at [http://localhost:3000](http://localhost:3000) and
-   sign up. The very first account created becomes the site's
-   administrator; everyone after that follows whatever
-   [sign-up mode](#sign-up-modes) is currently set (Open, by default).
+4. **Open the app** at [http://localhost:3000](http://localhost:3000) (or
+   whatever `APP_PORT` you set) and sign up. The very first account
+   created becomes the site's administrator; everyone after that follows
+   whatever [sign-up mode](#sign-up-modes) is currently set (Open, by
+   default).
 
 ### Updating
 
@@ -105,6 +106,7 @@ Docker Compose (see `.env.example` for the template).
 | `POSTGRES_USER`       | `db`                        | Postgres superuser created on first init.                                                                   |
 | `POSTGRES_PASSWORD`   | `db`                        | Password for `POSTGRES_USER`. Change this from the `.env.example` placeholder.                              |
 | `POSTGRES_DB`         | `db`                        | Database name created on first init.                                                                        |
+| `APP_PORT`            | `app` (host mapping only)   | Host port the app is published on, e.g. `3000` for `http://localhost:3000`. Defaults to `3000` if unset. The container always listens on 3000 internally regardless of this value. |
 | `DATABASE_URL`        | host-side tooling only      | Prisma connection string for running things like `npx prisma studio` from your own machine. **Not** used by the `app` container itself — see note below. |
 
 > **Note on `DATABASE_URL`:** inside `docker-compose.yml`, the `app`

docker-compose.yml

@@ -30,4 +30,7 @@ services:
       AUTH_SECRET: ${AUTH_SECRET}
       NODE_ENV: production
     ports:
-      - "3000:3000"
+      # Host-side port only -- the container itself always listens on
+      # 3000 internally (see Dockerfile's EXPOSE). Defaults to 3000 if
+      # APP_PORT isn't set.
+      - "${APP_PORT:-3000}:3000"