Fix Docker Compose setup

• Install wait-for-it with APT. The local version didn’t use to work, I guess because its permissions were overwritten by the COPY . /app instruction; and it was also legally questionable, as it was stored in this repo without the corresponding license file. Fortunately for us, Debian packages this script, so we can just install it with APT, which works (it installs the script to /bin, not to /app) and is definitely legal. Migrate the timeout change done in d9f8379d to a command-line parameter, which also highlights the difference much better than a random change in the middle of the file.
• Document the need to add a .env file (for the frontend). The file needs to be named .env, not .env.localhost, so update the .gitignore file to respect that.
• Set up proxying from the frontend to the backend to avoid CORS issues. This is only done in Docker mode, but it may also make sense in “manual” setup.