Starter from codebase

[joshdentremont]

Added ability to make starter with a different codebase.

In .env we can change the Packagist repo to something other than the starter site, or we can clone our own codebase folder and run make starter from that.

To test:

1. Run make starter as normal to get fresh starter site
2. make clean
3. Change the CODEBASE_PACKAGE variable in .env to another Packagist repo and run make starter to get a fresh install from that repo instead of the starter site
4. make clean
5. Put your own codebase folder in the isle-dc directory and run make starter. Instead of trying to pull from packagist it will just copy that into the container and run composer install

[aOelschlager]

I couldn't test the Packagist part because I don't have a codebase in there but I was able to test cloning a site from a github repository per your instructions and it ran great.