Y-websocket in docker image not working

I have used the Dockerfile in the y-websocket repo to create a y-webserver. The commands I used are:

git clone https://github.com/yjs/y-websocket.git && cd y-websocket
docker build -t micrology/y-websocket .
docker run -d -t -p1234:1234 --rm micrology/y-websocket

This should expose the server in the docker image on localhost:1234. However, when I run an app accessing this, I get (in the console):

WebSocket connection to 'ws://localhost:1234/prsmVSZ-CAD-QTE-ZMM' failed:

If, however, I abort the docker image and run y-websocket server in the terminal using

HOST=localhost PORT=1234 npx y-websocket

my app works as expected.

What am I doing wrong?

The solution is here: Can not visit the server in docker by the Dockerfile. · Issue #123 · yjs/y-websocket · GitHub The Dockerfile in the repo needs to have an extra line added: ENV HOST=