I don't quite understand how the add command in the container has anything to do with the previous rpc interface.

if I want to add a command such as ls, I will find the executable program in HOST, for example, mine is / usr/lib/klibc/ls

then copy the file to the container's rootfs, and the copy method searches for "container and host file sharing"

what does it mean to add a command?
usually you can get the command line by executing docker exec-it container name / container ID bash on the host, or if your basic image is alpine, you can also change bash to sh. Bash itself integrates common and commonly used commands, and generally does not require additional addition. Or if you want to add extra, you should use the package manager.