Enable supervisorctl usage

[alu-]

This pull request enables the RPC functionality of supervisord, making you able to control the processes that are managed by supervisord inside the application container. This pull request will also add a sub command to the sail shell script to interact with supervisorctl directly instead of going through a shell (./vendor/bin/sail shell or the likes) .

Example usage:
./vendor/bin/sail supervisorctl
./vendor/bin/sail supervisorctl restart php
./vendor/bin/sail supervisorctl --help

This change doesn't expose any additional ports as a UNIX socket is used. That socket is owned by the sail user so the security implications should be minimal, if any. The only noticeable change for the end user would be the new command in the sail shell script.

This will add value for those debugging the internals of the sail container and especially to those who have published the Dockerfiles and added their own programs to supervisord. It's also a possible timesave to have direct access towards the supervisorctl binary, and that it works out of the box.

Fixes #584

[taylorotwell]

You can publish the Docker files and add this if you wish.

[alu-]

You can publish the Docker files and add this if you wish.

Not the sail command, but otherwise correct. Thank you for taking the time reviewing this.