Building Custom Docker Images

Welcome back! In the previous lesson, you learned to write Dockerfiles. Now, let's turn those Dockerfiles into real Docker images and share them!

We'll cover how to:

• Use the docker build command
• Tag your images for versioning
• Log in to a Docker registry
• Push your custom images for others to use

The docker build command is your tool for creating Docker images from a Dockerfile and a "build context."

• It reads the instructions in your Dockerfile.
• It executes each instruction, creating a new layer in the image.
• Finally, it produces a new Docker image.

The basic syntax is docker build [OPTIONS] PATH | URL.

When you run docker build ., the . (dot) specifies the build context. This is the set of files at the specified PATH or URL that the Docker daemon can access during the build process.

• All files and folders in the build context are sent to the Docker daemon.
• This allows instructions like COPY . /app to work.
• Keep your build context clean to avoid sending unnecessary files!

Let's build a very simple image. Create a file named Dockerfile in an empty directory with the content below.

Then, open your terminal in that directory and try building the image:

To build the image from the Dockerfile you just created, run this command in your terminal:

docker build .

The . tells Docker to look for the Dockerfile in the current directory and use that directory as the build context.

After it completes, you'll see a new image ID!

Image tags are like labels for your images. They help you identify different versions or variants of an image.

• Tags are usually in the format name:tag (e.g., my-app:v1.0).
• The latest tag is often used for the most recent stable version.
• You can add a tag using the -t flag during the build process.

Let's rebuild our image, but this time, we'll give it a meaningful tag. Replace yourusername with your Docker Hub username.

After building, use docker images to see your newly tagged image!

A Docker Registry is a storage and distribution system for Docker images. Docker Hub is the most popular public registry.

To share your images or store them centrally, you'll push them to a registry:

1. First, log in to the registry (e.g., Docker Hub).
2. Then, use the docker push command.

Before pushing, you need to authenticate with Docker Hub (or another registry). This links your local Docker client to your account.

Run the following command in your terminal. You'll be prompted for your Docker Hub username and password:

Once logged in, you can push your image. Remember, the image tag must include your Docker Hub username (e.g., yourusername/my-app:v1.0).

Replace yourusername with your actual Docker Hub username:

You've learned to build and push images. Which command is used to assign a name and tag to an image during the build process?

Great job! You've mastered the core steps of turning a Dockerfile into a usable, shareable image:

• docker build .: Creates an image from your Dockerfile.
• docker build -t myuser/myimage:tag .: Builds and tags your image, preparing it for a registry.
• docker login: Authenticates you with a Docker registry like Docker Hub.
• docker push myuser/myimage:tag: Uploads your tagged image to the registry.

Now your custom applications can be packaged and distributed efficiently!