Build and run your image

• Orientation and setup
• Build and run your image
• Share images on Docker Hub

Prerequisites

Work through the orientation and setup in Part 1.

Introduction

Now that you’ve set up your development environment, you can begin to develop containerized applications. In general, the development workflow looks like this:

1. Create and test individual containers for each component of your application by first creating Docker images.

2. Assemble your containers and supporting infrastructure into a complete application.

3. Test, share, and deploy your complete containerized application.

In this stage of the tutorial, let’s focus on step 1 of this workflow: creating the images that your containers will be based on. Remember, a Docker image captures the private filesystem that your containerized processes will run in; you need to create an image that contains just what your application needs to run.

Set up

Let us download the node-bulletin-board example project. This is a simple bulletin board application written in Node.js.

• Git
• Windows (without Git)
• Mac or Linux (without Git)

git clone https://github.com/dockersamples/node-bulletin-board
cd node-bulletin-board/bulletin-board-app

curl.exe -LO https://github.com/dockersamples/node-bulletin-board/archive/master.zip
tar.exe xf master.zip
cd node-bulletin-board-master\bulletin-board-app

curl -LO https://github.com/dockersamples/node-bulletin-board/archive/master.zip
unzip master.zip
cd node-bulletin-board-master/bulletin-board-app

Define a container with Dockerfile

After downloading the project, take a look at the file called Dockerfile in the bulletin board application. Dockerfiles describe how to assemble a private filesystem for a container, and can also contain some metadata describing how to run a container based on this image.

For more information about the Dockerfile used in the bulletin board application, see Sample Dockerfile.

Build and test your image

Now that you have some source code and a Dockerfile, it’s time to build your first image, and make sure the containers launched from it work as expected.

Make sure you’re in the directory node-bulletin-board/bulletin-board-app in a terminal or PowerShell using the cd command. Run the following command to build your bulletin board image:

docker build --tag bulletinboard:1.0 .

You’ll see Docker step through each instruction in your Dockerfile, building up your image as it goes. If successful, the build process should end with a message Successfully tagged bulletinboard:1.0.

Windows users:

This example uses Linux containers. Make sure your environment is running Linux containers by right-clicking on the Docker logo in your system tray, and clicking Switch to Linux containers. Don’t worry - all the commands in this tutorial work the exact same way for Windows containers.

You may receive a message titled ‘SECURITY WARNING’ after running the image, noting the read, write, and execute permissions being set for files added to your image. We aren’t handling any sensitive information in this example, so feel free to disregard the warning in this example.

Run your image as a container

1. Run the following command to start a container based on your new image:

   docker run --publish 8000:8080 --detach --name bb bulletinboard:1.0
   

   There are a couple of common flags here:

   • --publish asks Docker to forward traffic incoming on the host’s port 8000 to the container’s port 8080. Containers have their own private set of ports, so if you want to reach one from the network, you have to forward traffic to it in this way. Otherwise, firewall rules will prevent all network traffic from reaching your container, as a default security posture.
   • --detach asks Docker to run this container in the background.
   • --name specifies a name with which you can refer to your container in subsequent commands, in this case bb.

2. Visit your application in a browser at localhost:8000. You should see your bulletin board application up and running. At this step, you would normally do everything you could to ensure your container works the way you expected; now would be the time to run unit tests, for example.

3. Once you’re satisfied that your bulletin board container works correctly, you can delete it:

   docker rm --force bb
   

   The --force option stops a running container, so it can be removed. If you stop the container running with docker stop bb first, then you do not need to use --force to remove it.

Conclusion

At this point, you’ve successfully built an image, performed a simple containerization of an application, and confirmed that your app runs successfully in its container. The next step will be to share your images on Docker Hub, so they can be easily downloaded and run on any destination machine.

On to Part 3 >>

Sample Dockerfile

Writing a Dockerfile is the first step to containerizing an application. You can think of these Dockerfile commands as a step-by-step recipe on how to build up your image. The Dockerfile in the bulletin board app looks like this:

# Use the official image as a parent image.
FROM node:current-slim

# Set the working directory.
WORKDIR /usr/src/app

# Copy the file from your host to your current location.
COPY package.json .

# Run the command inside your image filesystem.
RUN npm install

# Inform Docker that the container is listening on the specified port at runtime.
EXPOSE 8080

# Run the specified command within the container.
CMD [ "npm", "start" ]

# Copy the rest of your app's source code from your host to your image filesystem.
COPY . .

The dockerfile defined in this example takes the following steps:

• Start FROM the pre-existing node:current-slim image. This is an official image, built by the node.js vendors and validated by Docker to be a high-quality image containing the Node.js Long Term Support (LTS) interpreter and basic dependencies.
• Use WORKDIR to specify that all subsequent actions should be taken from the directory /usr/src/app in your image filesystem (never the host’s filesystem).
• COPY the file package.json from your host to the present location (.) in your image (so in this case, to /usr/src/app/package.json)
• RUN the command npm install inside your image filesystem (which will read package.json to determine your app’s node dependencies, and install them)
• COPY in the rest of your app’s source code from your host to your image filesystem.

You can see that these are much the same steps you might have taken to set up and install your app on your host. However, capturing these as a Dockerfile allows you to do the same thing inside a portable, isolated Docker image.

The steps above built up the filesystem of our image, but there are other lines in your Dockerfile.

The CMD directive is the first example of specifying some metadata in your image that describes how to run a container based on this image. In this case, it’s saying that the containerized process that this image is meant to support is npm start.

The EXPOSE 8080 informs Docker that the container is listening on port 8080 at runtime.

What you see above is a good way to organize a simple Dockerfile; always start with a FROM command, follow it with the steps to build up your private filesystem, and conclude with any metadata specifications. There are many more Dockerfile directives than just the few you see above. For a complete list, see the Dockerfile reference.

CLI references

Further documentation for all CLI commands used in this article are available here:

• docker image
• docker container
• Dockerfile reference