Alpine Docker Cron with Docker Compose

This tutorial provides a working example of configuring a cron job with an Alpine Docker image and Docker Compose. The example consists of a PHP application using php:8.2-fpm-alpine image and a cron job set to run every 5 minutes. The cron job prints a message to the log file when it completes. The following files are used:

docker-compose.yaml
Dockerfile
cronscript.sh
crontask-1.sh

File Overview

1. docker-compose.yaml

This file defines the php service using the Dockerfile for building the image. The command attribute is set to run the crond daemon in the foreground with log level 8.

version: "3.4"

services:
  php:
    build:
      context: .
      target: dev_tester
    command: sh -c "crond -f -l 8"

2. Dockerfile

This file builds the php:8.2-fpm-alpine image, creates a log file, and sets up the cron job with required permissions. It also copies the cronscript.sh and crontask-1.sh files to the appropriate locations.

FROM php:8.2-fpm-alpine AS dev_tester

RUN apk update && apk add --no-cache

ENV APP_ENV=prod
WORKDIR /srv/app

# Create the log file to be able to run tail
RUN touch /var/log/cron.log
RUN mkdir -p /etc/periodic/5min

COPY docker/scheduler/usr/local/cronscript.sh /usr/local/bin/cronscript.sh
COPY docker/scheduler/etc/periodic/5min/crontask-1.sh /etc/periodic/5min/crontask-1.sh
RUN chmod +x /usr/local/bin/cronscript.sh
RUN chmod +x /etc/periodic/5min/crontask-1.sh

# Add crontab entry
RUN echo "*/5       *       *       *       *       run-parts /etc/periodic/5min" > /etc/crontabs/root

CMD ["php-fpm"]

3. cronscript.sh

This script appends the cron job entry to the root crontab file and lists the crontab content.

#!/bin/sh
echo "*/5       *       *       *       *       run-parts /etc/periodic/5min" >> /etc/crontabs/root
crontab -l

4. crontask-1.sh

This script is the actual cron job that runs every 5 minutes. It writes a message to the log file and appends the current date and time when it completes.