Make crontab work

[DrPsychick]

Description of Issue

crontab feature was never implemented to support environment variables. As it looks, the container will never start cron because the crontab file never contains uncommented lines.

Suggestion

Support CRONTAB_xxxx variables to append lines to crontab file, before starting cron

for var in ${!CRONTAB_@}; do 
  eval 'echo "$'$var'" >> /ark/config/crontab'; 
done