This is often done via a super thin distribution like Alpine Linux to keep image size down, despite the COW functionality touted by Docker that's supposed to make it cheap to share layers.

The difference is that unlike a fully virtualized system, the container does not have to execute a full boot/init process; it executes only the process you request from within the container's image. Of course, one could request a process that starts many subservient services within the container, though that is typically considered bad form.

What people really want is super cheap VMs, but they're fooling themselves into believing they want containers, and pretending that containers are a magic bullet with no tradeoffs. It's scary times.

In my example, /j/postgres is that filesystem that can migrate anywhere. (What's actually started is /j/postgres/usr/local/bin/postgres.) Yeah, you can just specify the IP address when starting it.