rails, whenever and docker - cron tasks doesn't run - ruby-on-rails

Question

My crontasks from schedule.rb doesn't work on docker container, but crontab -l result already contains this lines:
# Begin Whenever generated tasks for: /app/config/schedule.rb
45 19 * * * /bin/bash -l -c 'bundle exec rake stats:cleanup'
45 19 * * * /bin/bash -l -c 'bundle exec rake stats:count'
0 5 * * * /bin/bash -l -c 'bundle exec rake stats:history'
# End Whenever generated tasks for: /app/config/schedule.rb
I can run this commands manually in container and it works. It seems like cron doesn't start.
Dockerfile:
FROM ruby:2.4.0-slim
RUN apt-get update
RUN apt-get install -qq -y --no-install-recommends build-essential libpq-dev cron postgresql-client
RUN cp /usr/share/zoneinfo/Europe/Moscow /etc/localtime
ENV LANG C.UTF-8
ENV RAILS_ENV production
ENV INSTALL_PATH /app
RUN mkdir $INSTALL_PATH
RUN touch /log/cron.log
ADD Gemfile Gemfile.lock ./
WORKDIR $INSTALL_PATH
RUN bundle install --binstubs --without development test
COPY . .
RUN bundle exec whenever --update-crontab
RUN service cron start
ENTRYPOINT ["bundle", "exec", "puma"]

Answer 1

In a Dockerfile, the RUN command is only execute when building the image.
If you want to start cron when you start your container, you should run cron in CMD. I modified your Dockerfile by removing RUN service cron start and changing your ENTRYPOINT.
FROM ruby:2.4.0-slim
RUN apt-get update
RUN apt-get install -qq -y --no-install-recommends build-essential libpq-dev cron postgresql-client
RUN cp /usr/share/zoneinfo/Europe/Moscow /etc/localtime
ENV LANG C.UTF-8
ENV RAILS_ENV production
ENV INSTALL_PATH /app
RUN mkdir $INSTALL_PATH
RUN touch /log/cron.log
ADD Gemfile Gemfile.lock ./
WORKDIR $INSTALL_PATH
RUN bundle install --binstubs --without development test
COPY . .
RUN bundle exec whenever --update-crontab
CMD cron && bundle exec puma
It's a best practice to reduce the number of layer an image have, for example, you should always combine RUN apt-get update with apt-get install in the same RUN statement, and clean apt files after: rm -rf /var/lib/apt/lists/*
FROM ruby:2.4.0-slim
RUN apt-get update && \
apt-get install -qq -y --no-install-recommends build-essential libpq-dev cron postgresql-client \
rm -rf /var/lib/apt/lists/* && \
cp /usr/share/zoneinfo/Europe/Moscow /etc/localtime
ENV LANG C.UTF-8
ENV RAILS_ENV production
ENV INSTALL_PATH /app
RUN mkdir $INSTALL_PATH && \
touch /log/cron.log
ADD Gemfile Gemfile.lock ./
WORKDIR $INSTALL_PATH
RUN bundle install --binstubs --without development test
COPY . .
RUN bundle exec whenever --update-crontab
CMD cron && bundle exec puma