> Edit: for clarity, by "multiple OS" I mean multiple Linux versions. Like if one project has a dependency on Ubuntu22 and another is easier with Ubuntu24. You don't have to stress "do I update my OS?"

For this part, I just create systemd-nspawn containers.

Last time I wanted to test something in a very old version of WebKit, creating a Debian Jessie container takes a few minutes. Things run at native speed.