Technology agnostic

As we've already seen it should be possible to containerize almost any project. As we are in between Dev and Ops let's pretend again that some developer teammates of ours did an application with a README that instructs what to install and how to run the application. Now we as the container experts can containerize it in seconds. Open this https://github.com/docker-hy/material-applications/tree/main/rails-example-project project and read through the README and think about how to transform it into a Dockerfile. Thanks to the README we should be able to decipher what we will need to do even if we have no clue about the language or technology!

We will need to clone the repository, which you may have already done. After the project is done, let's start with a Dockerfile. We know that we need to install ruby and whatever dependencies it had. Let's place the Dockerfile in the project root.

Dockerfile

# We need ruby 3.0.0. I found this from docker hub
FROM ruby:3.0.0

EXPOSE 3000

WORKDIR /usr/src/app

Ok these are the basics, we have FROM a ruby version, EXPOSE 3000 was told at the bottom of the README and WORKDIR /usr/src/app is the convention.

# Install node, found from the internet
RUN curl -sL https://deb.nodesource.com/setup_lts.x | bash -
RUN apt install -y nodejs

# Install yarn, found from readme
RUN npm install -g yarn

# Install the correct bundler version
RUN gem install bundler:2.2.11

Nodejs required a little bit of googling but that sounds promising. The next were told to us by the README. We won't need to copy anything from outside of the container to run these.

# Copy all of the content from the project to the image
COPY . .

# Install all dependencies
RUN bundle install

# We pick the production guide mode since we have no intention of developing the software inside the container.
# Run database migrations by following instructions from README
RUN rails db:migrate RAILS_ENV=production

# Precompile assets by following instructions from README
RUN rake assets:precompile

# And finally the command to run the application
CMD ["rails", "s", "-e", "production"]

And finally, we copy the project, install all of the dependencies and follow the instructions in the README.

Ok. Let's see how well monkeying the README worked for us: docker build . -t rails-project && docker run -p 3000:3000 rails-project. After a while of waiting, the application starts in port 3000 in production mode.