I think docker is FOSS itself, they do have enterprise but docker is FOSS

I think it's because there is vc funding so they don't opensource it because of them but tbh I'm not sure if anyone really knows why. I'm guessing they plan on profiting from it in the future somehow

if you want to look more into the license look here

https://github.com/getumbrel/umbrel#-license

basically you're free to do most except try and make profit off the code which makes it not FOSS