I found a small error in the build script, when enabling docker options to build sound kernel modules and install ALSA.
The filename that is created to download the ALSA tar file is different than the filename that is passed to the untar command, causing the file to not be found and fail to copy ALSA to the correct location.
Once I fixed up that build script, it managed to build the sound kernel modules, install ALSA and work perfectly on my machine, allowing local sound output from a docker container, nice work!
PS: I was going to just make a pull request to fix this, but I couldn't find a github repo for it - is there a public repo for this container?