Code Reading - Learn ❤️ Docker

Published Thursday, June 15, 2017

Notes from Flatiron School engineering team’s code reading on using Docker with our core codebase.


  • Watch this Youtube playlist (~90 min)
  • Clone down existing Rails app (in our case, the Learn codebase)
  • Set up account on Docker Hub


What is Docker?

  • Tool for managing and creating containers

Why use Docker?

  • Maintain a stable, consistent environment
  • Encapsulate all pices of an app
  • Uniformity and consistency
  • Step towards better deployments and auto-scaling infrastructure (Swarm, Kubernetes)
  • Lighter than a VM (no hypervisor)
  • One of the more mature tools available


Docker image:

  • Pre-packaged layer
  • Can stack layers on each other
  • Result of running Dockerfile
  • Can create containers off of image

Docker container:

  • Running isolated container of your app’s code

Docker networking:

  • Built-in
  • Allows containers to talk to each other over same network

Docker volumes:

  • Take local files and put them into your container
  • Edit locally, see changes in container

Docker compose:

  • Toolset for coordinating containers

Docker Hub:

  • Store and access repositories (images)


Run an image: docker run -it ubuntu (creates container with interactive terminal)

Build Dockerfile: docker build -t wget . (Note: . means look for Dockerfile in current directory)

FROM ruby:2.3

RUN apt-get update -yqq \
  && apt-get install -yqq --no-install-recommends \

WORKDIR /usr/src/app
COPY Gemfile* ./
RUN bundle install
COPY . .

CMD rails server -b

Side note: rails server -b binds to all ports.

Workshop: Running Learn on Docker

  1. Create Dockerfile in app directory (app === build context)
  FROM ruby:2.2.2cat

  RUN apt-get update -yqq \
    && apt-get install -yqq --no-install-recommends \
    postgresql-client nodejs npm \
    && npm install gulp -g \
    # symlink nodejs -> node
    && ln -s /usr/bin/nodejs /usr/bin/node

  WORKDIR /var/app

  COPY Gemfile* ./
  RUN bundle install

  COPY package.json ./
  RUN npm install

  COPY . .
  EXPOSE 3000
  CMD rails server -b
  1. Optional: add .dockerignore file
  2. cat .gitignore >> .dockerignore
  3. Build image: docker build -t ironboard .
  4. Run into errors? Get image id (docker images), then run docker run -it <hash> bash to drop into image at that point.
  5. docker images to see image
  6. docker run -d --name app -p 3000:3000 -v $PWD:/var/app ironboard (port flag: -p, share volume: -v)
  7. docker ps to see running containers
  8. docker logs --follow to see stdout

Additional resources