If you wanted to know "can you use rsync?" (a technical yes/no question), perhaps you should have asked that, instead of asking "why use dropbox?" (a subjective preference question)?

To answer the question of "can you use rsync instead of dropbox?" - yes, it is "technically possible" to use rsync to transfer files between multiple devices.

I don't understand why you would ask this question though, because everything is "technically possible" if you ignore all the constraints that stop it from being possible...

You did not read as well. The only question I asked was if you could fake the ext4 using docker. Nothing else. The rest does not have a question mark.

You answered a question I did never ask.

I assume English is not your first language? If you say "X? But the real question is Y", then it's quite valid to assume that "X" was hypothetical, and the real question was "Y" :)