- Before starting the container, docker injects itself inside the container by mount binding the dockerd binary into /sbin/init
- Instead of running the user process directly inside the container, we run /sbin/init targetprocess [args...]
- When docker is run as /sbin/init (e.g. argv[0] == "/sbin/init"), then its own sys init code kicks in
- The sys init code will be responsible for setting up the process environment prior to its execution (setuid, networking, ...).
- Finally, docker's sys init will exec() the container's process, thus replacing itself with the target binary (which will be running as pid 1)
Solomon Hykes