For the rest of this tutorial, we will be working with a simple app that is running in Node.js. If you’re not familiar with Node.js, don’t worry. No real JavaScript experience is needed.

In order to build the application, we need to use a Dockerfile. A Dockerfile is simply a text-based script of instructions that is used to create a container image.

1. Create a file named Dockerfile in the same folder as the file package.json with the following contents.

# Build an image starting with the node:alpine
FROM node:alpine

# Make a directory called /app
RUN mkdir /app

# Add the current directory . into the path /app in the image
ADD . /app

# Set the working directory to /app
WORKDIR /app

# Update and Start
RUN npm update
CMD npm start

Please check that the file Dockerfile has no file extension like .txt. Some editors may append this file extension automatically and this would result in an error in the next step.

2. If you haven’t already done so, open a terminal and go to the app directory with the Dockerfile. Now build the container image using the docker build command.

$ docker build -t node-nevtik:1.0 .

This command used the Dockerfile to build a new container image. You might have noticed that a lot of “layers” were downloaded. This is because we instructed the builder that we wanted to start from the node:alpine image. But, since we didn’t have that on our machine, that image needed to be downloaded.

After the image was downloaded, we added in our application and used npm to update our application’s. The CMD directive specifies the default command to run when starting a container from this image.

Finally, the -t flag tags our image. Think of this simply as a human-readable name for the final image.

Since we named the image node-nevtik:1.0, we can refer to that image when we run a container.

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

Now that we have an image, let’s run the application. To do so, we will use the docker run command.

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

docker run -d --name node-nevtik-container -p 3000:3000 node-nevtik:1.0

What's mean the -d --name and -p flags? We’re running the new container named "node-nevtik-container" in “detached” mode (in the background) and creating a mapping between the host’s port 3000 to the container’s port 3000. Without the port mapping, we wouldn’t be able to access the application.

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

In this short section, we learned the very basics about building a container image and created a Dockerfile to do so. Once we built an image, we started the container and saw the running app.