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:

   ```console
   $ 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](http://localhost:3000).
   You should see your app.