Added cron via env variables

[jhoglin]

A simple way influenced by https://github.com/eea/eea.docker.postgres/tree/master/postgres#supported-environment-variables

[LongLiveCHIEF]

The only thing this is missing is a VOLUME definition in the Dockerfile for the output dir of the cron job. Anything that make a container non-idempotent should ideally have a volume assignments.

Aside from that, I like this one! 👍