Containerize an application
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.
To complete this guide, you’ll need the following:
- Docker running locally. Follow the instructions to download and install Docker.
- A Git client.
- An IDE or a text editor to edit files. Docker recommends using Visual Studio Code.
- A conceptual understanding of containers and images.
Get the app
Before you can run the application, you need to get the application source code onto your machine.
-
Clone the getting-started repository using the following command:
$ git clone https://github.com/docker/getting-started.git
-
View the contents of the cloned repository. Inside the
getting-started/app
directory you should seepackage.json
and two subdirectories (src
andspec
).
Build the app’s container image
To build the container 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.
-
In the
app
directory, the same location as thepackage.json
file, create a file namedDockerfile
. You can use the following commands below to create a Dockerfile based on your operating system.In the terminal, run the following commands listed below.
Change directory to the
app
directory. Replace/path/to/app
with the path to yourgetting-started/app
directory.$ cd /path/to/app
Create an empty file named
Dockerfile
.$ touch Dockerfile
In the Windows Command Prompt, run the following commands listed below.
Change directory to the
app
directory. Replace\path\to\app
with the path to yourgetting-started\app
directory.$ cd \path\to\app
Create an empty file named
Dockerfile
.$ type nul > Dockerfile
-
Using a text editor or code editor, add the following contents to the Dockerfile:
# syntax=docker/dockerfile:1 FROM node:18-alpine WORKDIR /app COPY . . RUN yarn install --production CMD ["node", "src/index.js"] EXPOSE 3000
Note
Select an instruction in the Dockerfile example to learn more about the instruction.
-
Build the container image using the following commands:
In the terminal, change directory to the
getting-started/app
directory. Replace/path/to/app
with the path to yourgetting-started/app
directory.$ cd /path/to/app
Build the container image.
$ docker build -t getting-started .
The
docker build
command uses the Dockerfile to build a new container 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 thenode:18-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. TheCMD
directive specifies the default command to run when starting a container from this image.Finally, the
-t
flag tags your image. Think of this simply as a human-readable name for the final image. Since you named the imagegetting-started
, you can refer to that image when you run a container.The
.
at the end of thedocker build
command tells Docker that it should look for theDockerfile
in the current directory.
Start an app container
Now that you have an image, you can run the application in a container. To do so, you will use the docker run
command.
-
Start your container using the
docker run
command and specify the name of the image you just created:$ docker run -dp 3000:3000 getting-started
You use the
-d
flag to run the new container in “detached” mode (in the background). You also use the-p
flag to create a mapping between the host’s port 3000 to the container’s port 3000. Without the port mapping, you wouldn’t be able to access the application. -
After a few seconds, open your web browser to http://localhost:3000. You should see your app.
-
Go ahead and 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 should have a running todo list manager with a few items, all built by you.
If you take a quick look at your containers, you should see at least one container running that is 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 following 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 0.0.0.0:3000->3000/tcp priceless_mcclintock
In Docker Desktop, select the Containers tab to see a list of your containers.
Next steps
In this short section, you learned the basics about creating a Dockerfile to build a container image. Once you built an image, you started a container and saw the running app.
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.