HMR failing with Docker even when ports specified & exposed?

[mrcoles]

I cannot get HMR to work with my Docker setup. I created a simple test repo that reproduces the problem: https://github.com/mrcoles/simple-parcel-site

Even though I specify the --hmr-port and expose it via ports in my docker-compose.yml file, I encounter the error:

WebSocket connection to 'ws://localhost:1239/' failed:
undefined
[parcel] 🚨 Connection to the HMR server was lost

Does anyone know how I can further debug what’s going on here—or is this a bug?

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Also, here are screenshots from the “Network” tab in Chrome dev tools of the websocket (taken from a slightly different setup that uses port 8235):

Network tab on Docker setup:

Network tab on local filesystem setup:

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Edit: I updated to with the sample repo to reproduce things and made the question more specific.

[mrcoles]

I opened a bug report issue: #8966