Hi Evan, for what it is worth, I have found in the last few years, 3cx works just fine inside a vanilla LXC Container setup (ie, I have a ~dozen instances of 3cx running inside LXC on proxmox). Not exactly docker, but more adjacent to docker than bare metal for sure.
I am guessing the baseline from 3cx will not change though in terms of "This is what we do support in terms of hardware platform" vs "we don't support edge cases, good luck with that if you go that route you are on your own for support". Which is fair.
That being said, I have zero problems with 3cx on LXC unprivileged containers on proxmox. It is a straightforward workload that 'just works fine' for me in this kind of setup. A single modern 'entry level' server (ie, 8+ cores and 32+gigs ram) can support a bunch of 3cx instances without breaking a sweat. This is not exactly IO and CPU intensive workload.
In theory you can set yourself up with some kind of DIY 3cx-on-whatevr-hypervisor just by following online hints about how to get 3cx running on top of vanilla debian. Clearly it will be an 'unsupported' platform from 3cx perspective though.
Tim