Containerize an application

Table of contents

For the rest of this guide, you'll be working with a simple todo list manager that runs on Node.js. If you're not familiar with Node.js, don't worry. This guide doesn't require any prior experience with JavaScript.

Prerequisites

Get the app

Before you can run the application, you need to get the application source code onto your machine.

  1. Clone the getting-started-app repository using the following command:

    $ git clone https://github.com/docker/getting-started-app.git

  2. View the contents of the cloned repository. You should see the following files and sub-directories.

    ├── getting-started-app/
│ ├── .dockerignore
│ ├── package.json
│ ├── README.md
│ ├── spec/
│ ├── src/
│ └── yarn.lock

Build the app's image

To build the image, you'll need to use a Dockerfile. A Dockerfile is simply a text-based file with no file extension that contains a script of instructions. Docker uses this script to build a container image.

  1. In the getting-started-app directory, the same location as the package.json file, create a file named Dockerfile with the following contents:

    # syntax=docker/dockerfile:1

FROM node:lts-alpine
WORKDIR /app
COPY . .
RUN yarn install --production
CMD ["node", "src/index.js"]
EXPOSE 3000

    This Dockerfile starts off with a node:lts-alpine base image, a light-weight Linux image that comes with Node.js and the Yarn package manager pre-installed. It copies all of the source code into the image, installs the necessary dependencies, and starts the application.

  2. Build the image using the following commands:

    In the terminal, make sure you're in the getting-started-app directory. Replace /path/to/getting-started-app with the path to your getting-started-app directory.

    $ cd /path/to/getting-started-app

    Build the image.

    $ docker build -t getting-started .

    The docker build command uses the Dockerfile to build a new image. You might have noticed that Docker downloaded a lot of "layers". This is because you instructed the builder that you wanted to start from the node:lts-alpine image. But, since you didn't have that on your machine, Docker needed to download the image.

    After Docker downloaded the image, the instructions from the Dockerfile copied in your application and used yarn to install your application's dependencies. The CMD directive specifies the default command to run when starting a container from this image.

    Finally, the -t flag tags your image. Think of this as a human-readable name for the final image. Since you named the image getting-started, you can refer to that image when you run a container.

    The . at the end of the docker build command tells Docker that it should look for the Dockerfile in the current directory.

Start an app container

Now that you have an image, you can run the application in a container using the docker run command.

  1. Run your container using the docker run command and specify the name of the image you just created:

    $ docker run -d -p 127.0.0.1:3000:3000 getting-started

    The -d flag (short for --detach) runs the container in the background. This means that Docker starts your container and returns you to the terminal prompt. Also, it does not display logs in the terminal.

    The -p flag (short for --publish) creates a port mapping between the host and the container. The -p flag takes a string value in the format of HOST:CONTAINER, where HOST is the address on the host, and CONTAINER is the port on the container. The command publishes the container's port 3000 to 127.0.0.1:3000 (localhost:3000) on the host. Without the port mapping, you wouldn't be able to access the application from the host.

  2. After a few seconds, open your web browser to http://localhost:3000. You should see your app.

    Empty todo list

  3. Add an item or two and see that it works as you expect. You can mark items as complete and remove them. Your frontend is successfully storing items in the backend.

At this point, you have a running todo list manager with a few items.

If you take a quick look at your containers, you should see at least one container running that's using the getting-started image and on port 3000. To see your containers, you can use the CLI or Docker Desktop's graphical interface.

Run the docker ps command in a terminal to list your containers.

$ docker ps

Output similar to the following should appear.

CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                      NAMES
df784548666d        getting-started     "docker-entrypoint.s…"   2 minutes ago       Up 2 minutes        127.0.0.1:3000->3000/tcp   priceless_mcclintock

In Docker Desktop, select the Containers tab to see a list of your containers.

Docker Desktop with get-started container running

Summary

In this section, you learned the basics about creating a Dockerfile to build an image. Once you built an image, you started a container and saw the running app.

Related information:

Next steps

Next, you're going to make a modification to your app and learn how to update your running application with a new image. Along the way, you'll learn a few other useful commands.

Update the application