• To successfully follow this guide you will first need to install Docker on your local machine.
  • You will also need a Docker Hub account, which includes 1 free private repository.


  • We will be using Docker Hub throughout this learn guide, but GitLab (including the self-hosted version) comes with a built-in container registry for unlimited repositories and is a great alternative.

Create a Docker Image

Create a Dockerfile in the root of your project.

# We're currently using Ruby 2.5.1
FROM ruby:2.5.1-alpine

# Always nice to know who to contact
LABEL maintainer=""

# In a single step - install the dependences (apk is Alpine Linux's 
# equivalent to apt) and then remove the cached files (to keep the image 
# as small as we can)
RUN apk add --update bash build-base nodejs mariadb-dev tzdata && \
  rm -rf /var/cache/apk/*

# Make a directory to hold our app and set the working directory to it
RUN mkdir /app

# Do the gem installation stuff first, in a separate step, because they
# Don't change as often as the code (so we can cache them in a separate 
# Docker image layer)
COPY Gemfile Gemfile.lock ./
RUN bundle install --binstubs

# Now copy the current folder to the WORKDIR and drop root privileges
COPY . .
RUN chown -R nobody:nogroup /app

# Compile all assets
RUN bundle exec rake assets:precompile

# Inform Docker what port our container listens on at runtime

# Set a default command to start the Rails server (can override this for workers, etc.)
CMD ["bundle", "exec", "rails", "server", "-p", "3000"]

Build the Docker image and tag it <your_username>/<repo_name>:<tag>.

docker build -t <your_username>/k8s-sample-app:v0.1 .

Once this completes you can list all images using the docker images command:

$ docker images
REPOSITORY                               TAG                 IMAGE ID            CREATED             SIZE
<your_username>/k8s-sample-app           v0.1                104edfbbe440        2 minutes ago       617MB

Push to Docker Hub

To create a remote private repository, sign into Docker Hub, click on Repositories then Create Repository.

Log in via your local command line (to allow you to push):

docker login
# enter Docker Hub username and press enter
# enter Docker Hub password and press enter

Push the image to Docker Hub:

docker push <your_username>/k8s-sample-app:v0.1

Releasing a new version

After you've made some changes to your Rails app you will need to rebuild the image (ideally with a new tag) and then push it to Docker Hub.

docker build -t <your_username>/k8s-sample-app:v0.2 .
docker push <your_username>/k8s-sample-app:v0.2