Crontab in Docker for Rails

This approach allows to run cron tasks of Rails application usually maintained by Whenever gem.

Dockerfile with Cron

FROM ruby:2.4.3-slim-stretch

RUN apt-get update -qq && apt-get install --no-install-recommends -y \
    build-essential \
    patch \
    ruby-dev \
    zlib1g-dev \
    liblzma-dev \
    libxml2 \
    libxml2-dev \
    libpq-dev \
    default-libmysqlclient-dev \
    imagemagick \
    git \
    wget \
    gnupg2 \
    curl \
    nodejs-dev \
    node-gyp \
    libssl1.0-dev \
    cron \
    && wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | apt-key add - \
    && echo "deb http://apt.postgresql.org/pub/repos/apt/ stretch"-pgdg main | tee  /etc/apt/sources.list.d/pgdg.list \
    && apt-get update -qq && apt-get install --no-install-recommends --no-upgrade -y \
    postgresql-client-11 \
    && rm -rf /var/lib/apt/lists/*

RUN curl -fsSLO --compressed "https://nodejs.org/dist/v8.17.0/node-v8.17.0-linux-x64.tar.xz" \
  && tar -xJf "node-v8.17.0-linux-x64.tar.xz" -C /usr/local --strip-components=1 --no-same-owner \
  && rm "node-v8.17.0-linux-x64.tar.xz" \
  && ln -s /usr/local/bin/node /usr/local/bin/nodejs \
  && npm install -g yarn

WORKDIR /opt/app
COPY ./config/entrypoint.sh /usr/bin/entrypoint.sh
RUN chmod +x /usr/bin/entrypoint.sh && crontab -l | { cat; echo ""; } | crontab -

RUN gem install bundler -v 2.0.1

ADD Gemfile* /opt/app/
RUN bundle install

ARG RAILS_ENV
ARG ACTION_CABLE_URL

COPY . /opt/app

RUN yarn install --check-files
RUN RAILS_ENV=$RAILS_ENV bundle exec rake assets:precompile

EXPOSE 9292

The docker-compose.yml part to start container with cron

version: "3"
services:
  crontab:
    image: ${PROJECT_HUB}/${PROJECT_NAME}/backend:${IMAGE_TAG}
    container_name: ${PROJECT_NAME}_crontab
    restart: on-failure
    networks:
      overlay:
        aliases:
          - crontab
    working_dir: /opt/app
    env_file:
    - ./.env
    command: bash -c 'env > /opt/app/cronenv && bundle exec whenever --update-crontab && cron -f'
    depends_on:
      - redis
      - db

This file is required to run cron tasks in the same enviroment. We create it on startup.

    env > /opt/app/cronenv

Setup whenever to generate tasks with following additions:

* * * * * /bin/bash -l -c 'for line in `cat /opt/app/cronenv` ; do export $line ; done && cd /opt/app && RAILS_ENV=production bundle exec rake addons:process_repeat_addons >> /var/log/cron.log 2>&1'

The approach is a bit naive, but whenever does not allow to edit command before /bin/bash, otherwise we can do as follows:

* * * * * env - `cat /opt/app/cronenv` /bin/bash -l -c 'cd /opt/app && bundle install && RAILS_ENV=production bundle exec rake addons:process_repeat_addons >> /var/log/cron.log 2>&1'