Solved it and wrote the following comment elsewhere:
Notice that the docker file is automatically installed for amd64 architectures. If you have an arm64 system you’ll get “standard_init_linux.go:211: exec user process caused “exec format error””. The repo also had a problem with fixuid which was not supporting arm64 until the latest revision. I’ve got this to work with the very latest rc. Simply change the last part of docker run call to codercom/code-server:3.3.0-rc.7-arm64 and make sure to check the current releases for the latest (ideally officially stable) version https://hub.docker.com/r/codercom/code-server/tags