Make rails and sidekiq work together from different Docker containers (but can't use docker-compose)

I'm moving a rails app from Heroku to a linux server and deploying it using Caprover. It's an app very dependent on background jobs, which I run with sidekiq.

I've managed to make it work by running both the rails server (bundle exec rails server -b 0.0.0.0 -p80 &) and sidekiq (bundle exec sidekiq &) from a script that launches both in the CMD of the Dockerfile.

But I guess it would be much better (separation of concerns) if the rails server was in one Docker container and sidekiq in another one. But I can't figure out how to connect them. How do I tell my rails app that sidekiq lives in another container?

Because I use Caprover I'm limited to Dockerfiles to deploy my images, so I can't use docker-compose.

Is there a way to tell rails that it should use a certain sidekiq found in a certain Docker container? Caprover uses Docker swarm if that is of any help.

Am I thinking about this the wrong way?

My setup, currently, is as follows:

• 1 Docker container with rails server + sidekiq
• 1 Docker container with the postgres DB
• 1 Docker container with the Redis DB

My desired setup would be:

• 1 Docker container with rails server
• 1 Docker container with sidekiq
• 1 Docker container with postgres DB
• 1 Docker container with Redis DB

Is that even possible with my current limitations?

My rails + sidekiq Dockerfile is as follows:

FROM ruby:2.6.4-alpine
#
RUN apk update && apk add nodejs yarn postgresql-client postgresql-dev tzdata build-base ffmpeg
RUN apk add --no-cache --upgrade bash
RUN mkdir /myapp
WORKDIR /myapp
COPY Gemfile /myapp/Gemfile
COPY Gemfile.lock /myapp/Gemfile.lock
RUN bundle install --deployment --without development test
COPY . /myapp
RUN yarn 
RUN bundle exec rake yarn:install
# Set production environment
ENV RAILS_ENV production
ENV RAILS_SERVE_STATIC_FILES true
# Assets, to fix missing secret key issue during building
RUN SECRET_KEY_BASE=dumb bundle exec rails assets:precompile
# Add a script to be executed every time the container starts.
COPY entrypoint.sh /usr/bin/
RUN chmod +x /usr/bin/entrypoint.sh
ENTRYPOINT ["entrypoint.sh"]
EXPOSE 80
COPY start_rails_and_sidekiq.sh /myapp/start_rails_and_sidekiq.sh
RUN chmod +x /myapp/start_rails_and_sidekiq.sh
# Start the main process.
WORKDIR /myapp
CMD ./start_rails_and_sidekiq.sh

the start_rails_and_sidekiq.sh looks like this:

#!/bin/bash

# Start the first process
bundle exec rails server -b 0.0.0.0 -p80 &
status=$?
if [ $status -ne 0 ]; then
  echo "Failed to start Rails server: $status"
  exit $status
fi

# Start the second process
bundle exec sidekiq &
status=$?
if [ $status -ne 0 ]; then
  echo "Failed to start Sidekiq: $status"
  exit $status
fi

# Naive check runs checks once a minute to see if either of the processes exited.
# This illustrates part of the heavy lifting you need to do if you want to run
# more than one service in a container. The container exits with an error
# if it detects that either of the processes has exited.
# Otherwise it loops forever, waking up every 60 seconds

while sleep 60; do
  ps aux |grep puma |grep -q -v grep
  PROCESS_1_STATUS=$?
  ps aux |grep sidekiq |grep -q -v grep
  PROCESS_2_STATUS=$?
  # If the greps above find anything, they exit with 0 status
  # If they are not both 0, then something is wrong
  if [ $PROCESS_1_STATUS -ne 0 -o $PROCESS_2_STATUS -ne 0 ]; then
    echo "One of the processes has already exited."
    exit 1
  fi
done

I'm totally lost! Thanks in advance!