...and it was probably around 1.5K lines early in its life.

But that misses the point. This is very helpful for understanding how a Docker-like system works since it's a small and mostly self-contained implementation. You can read through the entire thing and fully understand it.

It only cheats in the container registry handling where it pulls in github.com/google/go-containerregistry and for the network setup where it uses github.com/vishvananda/netlink. The rest is done in terms of Go stdlib and syscalls.

Early Docker used external utilities (lxc, iptables) and had the client/server stuff already so it's not as straightforward.