Like @jpetazzo explained in "Docker SSHD considered evil": containers run with only one executable as main process. So if you want to have a sshd next to your php-fpm, you will need monit or supervisor to launch them. That's already 3 services in total!
That's why: keep your docker image clean!
But how to enter the container? There is a little tool called "nsenter" (available with util-linux >= 2.24)