does anyone here use docker with webpacker?

No, but I would guess those trying to put it in a separate container are likely trying to keep build-time libraries out of the production runtime. Docker has a build stages feature which is relatively recent (last year or two) which is probably a good option if that’s something you’re wanting to achieve too

If you don’t wanna get fancy, there’s always installing npm direct into your ruby container to give you an ExecJS runtime (or whatever other npm tooling for example) - it does mean you probably should care more about patching both ruby and node in your container though