Custom Image Unix, Filesystem, and cloud-init Requirements

I was reading about custom images here (https://docs.digitalocean.com/products/images/custom-images/details/features/), and came across three requirements that I can’t seem to find more details on.

1. Operating system must be Unix-like. Just how Unix-like does my OS have to be? What in particular makes it necessary to have a Unix-like OS? What specific Unix features does Digital Ocean expect, and what does it use them for?

2. OS must support ext3 or ext4 file systems. Does this mean the OS needs to be on an ext fs, or just needs to read one? What exactly is ext required for? I’d prefer to have no filesystem at all if possible.

3. cloud-init is required. What exactly is cloud-init used for? The page linked says that droplets without it won’t function correctly, but it doesn’t provide details regarding why. I was reading through the metadata in the cloud-init drive on an Ubuntu droplet, and it seems to contain some network config info. As I understand it, custom images use DHCP to configure their network systems, so would any of the network config info even be relevant? There’s also some sort of droplet agent install script, but I’m not sure what this does exactly. What exactly do I need cloud-init for? If it is necessary for operation, is it feasible to replicate the necessary functionality without cloud-init?

I need my OS to do exactly two things: read/write network packets that can reach the internet and read/write from its disk. As long as I can achieve this, I shouldn’t need any further configuration.

This page also listed SSH as a requirement, but I don’t think that’s relevant to me, as I plan to access it over TLS.

Thanks for your time.