Fix docker development:

- Specify elixir version (1.15) in Dockerfile base image

- Add npm package to Dockerfile

- Fix vite server unreachable from outside the docker container:
  issue https://framagit.org/framasoft/mobilizon/-/issues/1225

docker-compose.yml

@@ -19,6 +19,7 @@ services:
       - ".:/app"
     ports:
       - 4000:4000
+      - 5173:5173
     depends_on:
       - postgres
     environment: