Dockerfile Basics With A Hands On Example

A Dockerfile is a text file that contains instructions for building a Docker image.

Instead of manually preparing a machine, you describe the setup once in a Dockerfile. Docker reads those instructions step by step and creates an image. That image can then be used to run containers.

This is one of the most important Docker ideas: a Dockerfile turns setup steps into repeatable deployment instructions.

What a Dockerfile does

A Dockerfile answers questions like:

• Which base image should we start from?
• Which folder should the app run inside?
• Which files should be copied into the image?
• Which commands should run while building the image?
• Which port does the application use?
• Which command should start the app?

Without Docker, these steps may be written in a README and followed manually. With Docker, they become executable instructions.

A simple Dockerfile

Create a folder with an index.html file:

<h1>Hello from Docker</h1>

Now create a file named Dockerfile in the same folder:

FROM nginx
COPY index.html /usr/share/nginx/html/index.html

This Dockerfile starts from the official nginx image and replaces the default web page with your own index.html.

Build the image

Run this command from the folder where the Dockerfile exists:

docker build -t my-nginx-page .

The -t option gives the image a name. The dot at the end means "use the current folder as the build context."

Check the image:

docker images

You should see my-nginx-page in the image list.

Run the image as a container

Start a container from the image:

docker run -d -p 8080:80 --name my-page my-nginx-page

Open this in your browser:

http://localhost:8080

Docker sends traffic from host port 8080 to container port 80, where Nginx is serving your page.

Dockerfile instructions are layered

Each Dockerfile instruction creates a layer in the image.

For example:

FROM nginx
COPY index.html /usr/share/nginx/html/index.html

Docker starts with the nginx image layer, then adds another layer for your copied file.

This layer system is why Docker can cache builds. If an instruction has not changed, Docker can reuse the existing layer instead of rebuilding everything.

Clean up

After testing, stop and remove the container:

docker stop my-page
docker rm my-page

The image still remains. You can remove it separately:

docker rmi my-nginx-page

Written By: Shiva Srivastava