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

Dockerfile

@@ -1,9 +1,10 @@
-FROM elixir:alpine
+FROM elixir:1.15-alpine
 
-RUN apk add --no-cache inotify-tools postgresql-client file make gcc libc-dev argon2 imagemagick cmake build-base libwebp-tools bash ncurses git python3
+RUN apk add --no-cache inotify-tools postgresql-client file make gcc libc-dev argon2 imagemagick cmake build-base libwebp-tools bash ncurses git python3 npm
 
 RUN mix local.hex --force && mix local.rebar --force
 
 WORKDIR /app
 
 EXPOSE 4000
+EXPOSE 5173