You should be fine with just Godot for now. Don't over-engineer things until you need to, or you'll just scare away potential contributors.
If you need to test portability, try spinning up a VM or a second system and getting the project running on that new system. You can work out the issues from there.
A final thought: one method for open sourcing commercial games you may want to consider is separating the assets and engine into multiple repositories, with a restrictive (paid) license for the former. The engine can then be openly developed by owners of the game.
Of course, I'm gonna support anyone who open-sources the FULL game but it's worth considering if you're thinking of this as a hobby project or a commercial venture.