How to Docker

Did you just start a job and they run something called Docker? Is your company starting to “dockerize” their application? Are you just curious about what Docker is?

Well, here’s an intro 🤗.

What is Docker and why do I care?

What is Docker?

Docker is an open-source container technology

What is a container?

Container is an isolated sandbox/environment that packages up an application and all of its stuff (stack/dependencies/etc)

Basically, anything that the application needs is wrapped up in the container

Why is a container useful?

Abstracts the OS away from the application and solves the “but this works on my machine” problem

Why not use VM’s?

Containers are super light weight compared to VMs as containers uses parts of the hosts OS’s kernel

VMs VMs Look at the Guest OS layer, which is an entire operating system (read: heavy)

Containers Containers Look at how Docker replaces the Guest OS layer by virtualizing the OS and running containers (lightweight, baby) rather than full OSs

Docker

To get a feel of Docker, we’ll walk through running an app through Docker

  1. Clone the emoji-search repository and cd to the repo 
    git clone https://github.com/ahfarmer/emoji-search.git && cd emoji-search
  2. Run the app through a Docker container 
    docker run --rm -it -v $(pwd):/var/www/html -w /var/www/html -p 3000:3000 node:14 yarn install && yarn start
  3. Go to http://localhost:3000 and check out the app
  4. That’s it!

So, what’s going on? What are all the flags?

Here’s a breakdown of each argument

  • docker
    • The Docker CLI command
  • run
    • Run a specific image
  • --rm
    • Automatically clean up the container and file system when the container exits
  • -it
    • -i keeps STDIN open
    • -t allocates a pseudo-tty (basically a terminal)
  • -v host:container
    • Mounts a volume from from the host machine to the container
    • In this case, map the current working directory to /var/www/html
  • -w working/directory
    • Changes the working directory of the container to the specified path
    • In this case, make the working directory /var/www/html
  • -p container-port:host-port
    • Maps a container port to host port
    • In this case, map container port 3000 to host port 3000
  • node:14
  • yarn install && yarn start
    • Command for the container to run
    • In this case, install the dependencies and start the dev server

Dockerfile

The command above is pretty lengthy and annoying to type. To solve that, there are Dockerfiles.

A Dockerfile is basically a declarative way to automate image construction

Here’s how we would create a Dockerfile for the previous command

# Use the node:14 image from dockerhub
FROM node:14
 
# Change the working directory to /var/www/html
WORKDIR /var/www/html
 
# Copy the contents of the host's current directory to the working directory of the container
COPY . .
 
# Install the ndocker run --rm -it -v $(pwd):/var/www/html -w /var/www/html -p 3000:3000 node:14 yarn install && yarn startode dependencies
RUN yarn install
 
# Expose the dev server port
# Note: This doesn't actually publish the port
EXPOSE 3000
 
# Start the dev server
CMD ["yarn", "start"]

Now build that image by running the following

docker build -t emoji-search .

Here’s a breakdown of the command

  • docker
    • docker CLI command
  • build
    • Build a new image
  • -t name-of-image
    • Name the image
    • In this case, emoji-search
  • .
    • Directory with a Dockerfile
    • In this case, the current directory

Now, try running the following

docker run --rm -it -p 3000:3000 emoji-search

You should be able to access the web app on http://localhost:3000

Docker Compose

Docker Compose is a general tool to help define and run multi-container Docker applications. It also lets you run services with docker-compose up, which is as easy as it gets

Create a file called docker-compose.yml with the following contents

# Select a version of docker-compose 
# 3 is currently the latest 
# Here are the docs if you want to learn more https://docs.docker.com/compose/compose-file/compose-versioning/
version: '3'

# List of services that make of the application
services:
  # The name of the service
  emoji-search:
    # Where to find the Dockerfile
    build: .
    # Map ports from container to host
    ports:
      - 3000:3000

Now run the following

docker-compose up

You should be able to access the web app on http://localhost:3000

Useful commands

Sometimes you’ll want to run commands inside a sandboxed version of the container. In that case, you can use the docker-compose run command:

docker-compose run --rm [service-name] [command]

In other cases, you’ll might want to “ssh” directly into the running container. To do that, you’ll run docker-compose exec

docker-compose exec [service-name] [command]

Notes

Technology changes super quickly so it can be pretty important to know what versions of software are running. Here’s my build at the time I wrote this post:

16.04.1-Ubuntu
Docker version 18.03.0-ce, build 0520e24
docker-compose version 1.17.0, build ac53b73