Simple docker cron sidecar container based on alpine image

cronscript.sh

#!/usr/bin/env sh
echo "Job started: $(date)"
# Add additional instructions
echo "Job finished: $(date)"

Dockerfile

FROM alpine:3.8

# Alpine comes with built in cron schedules
# min   hour    day     month   weekday command
# */15    *       *       *       *       run-parts /etc/periodic/15min
# 0       *       *       *       *       run-parts /etc/periodic/hourly
# 0       2       *       *       *       run-parts /etc/periodic/daily
# 0       3       *       *       6       run-parts /etc/periodic/weekly
# 0       5       1       *       *       run-parts /etc/periodic/monthly

# Place script in appropriate folder
COPY cronscript.sh /etc/periodic/15min/backup

# bash-4.4# crond --help
# BusyBox v1.28.4 (2018-05-30 10:45:57 UTC) multi-call binary.
# Usage: crond -fbS -l N -d N -L LOGFILE -c DIR
#        -f      Foreground
#        -b      Background (default)
#        -S      Log to syslog (default)
#        -l N    Set log level. Most verbose 0, default 8
#        -d N    Set log level, log to stderr
#        -L FILE Log to FILE
#        -c DIR  Cron dir. Default:/var/spool/cron/crontabs 

# This runs cron in the foreground with loglevel 2
CMD [ "crond", "-l", "2", "-f" ]

FROM php:fpm-alpine
...
COPY cronscript.sh /etc/periodic/15min/backup

RUN dos2unix /etc/periodic/15min/backup

WORKDIR /var/www

CMD composer install ; wait-for-it database:3306 -- bin/console doctrine:migrations:migrate ;

CMD [ "crond", "-l", "2", "-f" ]

EXPOSE 9000

In my case i tried to add your cron inside a php:fpm-alpine Dockerfile

Then i built and i runned the container :

winpty docker exec -ti 171c18ba2257 sh

How can i receive the cron message ?

FROM php:fpm-alpine
...
COPY cronscript.sh /etc/periodic/15min/backup

RUN dos2unix /etc/periodic/15min/backup

WORKDIR /var/www

CMD composer install ; wait-for-it database:3306 -- bin/console doctrine:migrations:migrate ;

CMD [ "crond", "-l", "2", "-f" ]

EXPOSE 9000

In my case i tried to add your cron inside a php:fpm-alpine Dockerfile

Then i built and i runned the container :

winpty docker exec -ti 171c18ba2257 sh

How can i receive the cron message ?

I do not understand your question. Do you want to see the cron scheduled script output? If so, it should be displayed directly in the docker logs.

How can we start the crond with non root user ?