Running a Docker Image

What is a Docker Image?

A Docker image is a collection of self-contained files that act as a template for creating containers. Images can be seen as frozen or read-only containers that are exchanged through the Docker registry. When you download an image from the Docker registry and spin up a container from the downloaded image, you are essentially creating a writable layer on top of the existing read-only layers bundled with the image.

Running a Base Nginx Image

Nginx is an open-source web server, that is now also used a reverse proxy, HTTP cache, and a load balancer. Nginx was built with a focus on high performance and low memory usage.

Create a base container using the official Nginx image:
```
 docker create --name nginx_base -p 80:80 nginx:alpine
```
Note that the create command creates the container but does not actually start it. You can verify that the nginx_base is not running using the docker container ls command.
Verify that the image was successfully created by listing all existing images using the docker images -a command.
Start the nginx_base container:
```
 docker start nginx_base
```
Visit http://localhost on your browser and you should be presented with the Nginx landing page.
Modify the container to display a custom index page:
1. Create an index.html file on your machine using the editor of your choice.
2. Paste the following HTML code into the index.html file.
```
 <html>
 <head>
 <title>Hello World</title>
 </head>
 <body>
 <h1>Hello World!</h1>
 </body>
```
3. Use the docker cp command to copy the file over to the running nginx_base container:
```
 docker cp index.html nginx_base:/usr/share/nginx/html/index.html
```
4. Reload your browser and you should presented with a new updated “Hello, World” page you just created!
Create an image from your container using the following command:
```
 docker commit nginx_base
```
Run the docker images -a command and verify that there is a new image that was created recently. The image will not have a repository or tag.
Tag the image:
```
 docker tag <Image ID> my_image
```
Make sure to replace <Image ID> with your own image’s ID found by running the docker images -a command. In this example, my_image is the tag name that we chose for this image. Feel free to pick any tag name that conforms to the naming convention (i.e., a tag name must be valid ASCII and may contain lowercase and uppercase letters, digits, underscores, periods and hyphens. A tag name may not start with a period or a hyphen and may contain a maximum of 128 characters.).

After tagging the image, run the docker images -a command again and verify that the image now has a tag.
Run the docker container ls command and verify that the original container that you started in Step 3 is still running.
Stop the original container:
```
 docker stop nginx_base
```
You can now create and run containers from the image you created. As previously discussed, you can create and run containers from an image in one command:
```
docker run --name my_new_container -d -p 80:80 my_image
```

In the next section, we will detail how to create a Docker image for a Next.js website serving from a Nginx server using Docker Compose.