* Update to php:7.3-apache base docker image
* Update Grav version to 1.6.16
Add missing libzip-dev dependency.
* Add persistent volume
* Add cron support
* Add vi editor
Allows editing files when using `docker exec -it <container> /bin/bash`
* Correct vi editor install
* Updated README.md for version 1.6.16
Since the data volume is automatically persisted removed the extra information regarding volume mapping.