I ran into this same problem. I have a docker container that runs cron to execute some shell scripts periodically. I too had a hard time finding out why my scripts would run fine when I manually executed them inside the container. I tried all the tricks of creating a shell script that would run first to set the environment, but they never worked for me (most likely I did something wrong). But I continued looking and found this and it does work.
- Setup a start or entry point shell script for your cron container
- Make this the first line to execute
printenv | grep -v "no_proxy" >> /etc/environment
The trick here is the /etc/environment file. When the container is built that file is empty, I think on purpose. I found a reference to this file in the man pages for cron(8). After looking at all the versions of cron they all elude to an /etc/? file that you can use to feed environment variables to child processes.
Also, note that I created my docker container to run cron in the foreground, cron -f. This helped me avoid other tricks with tail running to keep the container up.
Here is my entrypoint.sh file for reference and my container is a debian:jessie base image.
printenv | grep -v "no_proxy" >> /etc/environment
cron -f
Also, this trick worked even with environment variables that are set during, docker run commands.