HI just a hint, usually when this happens in a cron is because it needs the full path to where docker is installed, so give it the complete path to where docker installed which could be /var/lib/docker depending on the distro.
We specialize on co-located teams in South America with constant training and we aim for indefinite long term business relationships, we work by providing full time programmers, but this is project is small enough that we can quote it.
Sincerely
Pablo V
Atlanticsoft