Suppose you’re building a feature that requires $newlib as a dependency. How is that handled? Do you request the addition of $newlib and wait for a new remote image to be deployed for everyone before you can work on your feature branch?

IME these remote instances come with sudo rights, and one is free to install/configure as they please. The initial image can be chosen too.

Right. The devs have full sudo access. They can install and configure whatever they want. Our Bento tool has pre/post setup hooks for all services. So teams can automate this. When something is commonly used we just preinstall it for them. Or some just contribute to the repo. It is all fairly straight forward Ansible and shell scripts. We push nightly to everyone's existing machines.