invoke-mf2pt1.sh: 7: python: not found

[Dan Eble]

I'm trying to upgrade the LilyDev Dockerfile to use Ubuntu 20.04 and I get this 
error when I try to build.  It's coming from here:

    # realpath doesn't exist on OSX
    realpath() {
      python -c "import os; print(os.path.realpath('$1'))"
    }

Is there a Right Way to resolve this?  Should this call python3?  Should I 
change the Docker image, say by creating a symlink?  TIA.
— 
Dan