Add new make command to open up a shell in the PHP container

[rostockahoi]

Not much more to say about that 🤷

[khalwat]

Thanks for the PR! I'm going to make some minor changes to this after merging it in, primarily to change it to make ssh (it's a convention I've been using elsewhere, and also the general paradigm is that I'm suffixing make with what you'd normally type to do the thing).

Also the shell command probably should be /bin/sh, and it probably should not pass along the additional commands to the container.