How Dockerfile Works?

You can deploy only a single container with the help of the docker image. However, if you need to deploy several containers (each for different tasks) from the same image then what happens? You can resolve this with the help of the dockerfile.

Dockerfile is a simple text file that contains all the commands user could call on the command line to assemble or build an image. With the help of docker build, you can easily automate build that runs multiple commands defines in the dockerfile in succession.

For example, you want to download the docker image from the docker hub for your specific development needs. Then, you want to update the image, install some packages for your development process. In this case, you can create a dockerfile, include all parameters for your specific needs, and build your custom image. After creating a dockerfile, you can use it over and over to build the same image without manually installing all required packages for your development process.

Dockerfile Basic

Before working with dockerfile if is important to know how to make a dockerfile. Dockerfile includes specific keywords that can be used to build a specific image. A brief explanation of all keywords used in a dockerfile are listed below:

• FROM: It is used to define the base image, on which we will be building.
• ADD: It is used to add files to the container being built. In simple terms, RUN is used to run commands and commits the result
• RUN: It is used to add layers to the base image, by installing components.
• CMD: It is used to run commands at the start of the container. These commands run only when there is no argument specified while running the container.
• ENTRYPOINT: It is used to run commands during the container initialization process. You must use an ENTRYPOINT in your dockefile if you want to start a container automatically after building an image.
• ENV: It is used to define environment variables in the container run-time.
• EXPOSE: It is used to specify the listening port to enable networking at run time.
• MAINTAINER: It is used to specify the name and email id of the image creator.
• USER: It is used to specify the username used to run the container.
• VOLUME: It is used to allow access from the container to the directory on the Docker host.
• WORKDIR: It is used to specify the path of the command to be executed at run time.
• LABEL: It is used to add labels to the docker image.

Creating the Dockerfile

In this section, we will create a dockerfile to build a LAMP server image from the Ubuntu base image.

First, you will need to create a directory to store the dockerfile. You can create it with the following command:

Next, create a directory named Dockerfile inside the directory:

Add the following lines:

Save and close the file when you are finished.

Building Image with Dockerfile

After creating the Dockerfile, you can easily create a custom LAMP image with the help of the Dockerfile.

First, change the directory to LAMP and run the following command to build the image from that file:

The above command will start downloading the Ubuntu latest image from the Docker Hub and installed necessary packages specified in the Dockerfile. Once the image has been built successfully, you should see the following output:

You can now list your newly build an image by running the following command:

You should see the following output:

Now, you have a custom LAMP server image in your hand. You can also see the history of each command with the following command:

You should see the following output:

Creating a Container from LAMP Image

Now, you can run a container from your image using the following command:

Once the container has been started, you should see the following output:

You can verify the running container using the following command:

You should get the following output:

Conclusion

In this guide, you learned what is Dockerfile and how to build a docker image with Dockerfile. This is very useful for creating your custom images instead of committing changes to a pulled image.