Sure, but S3 is totally optional, you could as easily (if not moreso) sit Discourse behind Cloudflare and just aggressively cache the assets path. Assets live within the container filesystem, go read up on docker before you go any further, that sounds to be the fundamental gap in your understanding here.
What purpose would cpanel serve? You can do anything except fundamental configuration changes from /admin. If you aren’t conversant with running servers over SSH then there are hosted options too.
Backups contain everything in terms of content in a single tarball. If you don’t use any additional plugins then assuming two servers are running the same versions moving between them is seamless.
There’s plenty of documentation on nearly every aspect of Discourse from installing it the first time, manually updating it, installing plugins, creating your own plugins and much much more. Go check out HowTo.