Add CRONTAB_PROJECT to environment

[roverwolf]

• add environment substitution for the project field
• add a CRONTAB_PROJECT environment variable to allow reference to the project that the crontab container is running in

This will allow the project name in the crontab file to be "$CRONTAB_PROJECT" which will get replaced by the project that is running the crontab container.

Related to https://github.com/willfarrell/docker-crontab/issues/38

[kgiglia]

This works perfectly for me, thank you!