Then I created a Docker image with Linux, Gnome, and novnc so I can spin one up instantly with little resource overhead and control it from any web browser.
Sort of, Proxmox does use noVNC I think, but it's a lot of overhead. This is just a docker command. I've finally put a page up for it: https://nowsci.com/webbian/
Correct me I'd I'm wrong, but with docker you're limited to the filesyatems and the image of the OS you're installing. If you need to experiment with the pre-OS boot events, can that even be accomplished with docker? E.g., trying out different GRUB settings, setting up LUKS with dropbear etc. I think those things require a VM.
Yeah, you are correct. Docker shares the kernel with the host operating system, it doesn't use hardware virtualization. That's why it's so fast and simple, but it also means it's not a traditional VM and thus comes with some limitations.