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Hi I don't know how can I run a cron job inside this container.

I've found this: How to run a cron job inside a docker container

But that overrides the CMD, I don't know hot to keep php-fpm working

Answers

When you need to run multiple processes in your docker container a solution is to use supervisord as the main instruction. Docker will start and monitor supervisord which in turn will start your other processes.

Docker File Example:

FROM debian:9
...
CMD ["/usr/bin/supervisord", "-c", "/etc/supervisor/my.conf"]

Supervisord config example (/etc/supervisor/my.conf):

[supervisord]
nodaemon=true

[program:cron]
command=/usr/sbin/crond -f -l 8
stdout_logfile=/dev/stdout
stderr_logfile=/dev/stderr
stdout_logfile_maxbytes=0
stderr_logfile_maxbytes=0
autorestart=true

[program:php-fpm]
command=docker-php-entrypoint php-fpm

Note that it is desirable to configure supervisord to output the logs to /dev/stdout and /dev/stderr to allow docker to handle these logs. Otherwise you risk your container to slow down over time as the amount of file writes increases.