I appreciate the work on this container as I have been thinking of something like this for a while, and running the GUI in a VM and moving everything there sounds counterproductive.   Having spent some time with it, I wanted to add a little insight that may help from my debugging.   Since you're using ubuntu latest, I'm wondering if latest LTS has shifted since creating this container.  Have you tried building without cache for this recently? I ask because if I build and run jrei's con