how does docker build works?

The docker build command is used to create a Docker image from a set of instructions defined in a Dockerfile. The Dockerfile contains a sequence of commands and settings that specify how the image should be constructed. When you run docker build, Docker reads the Dockerfile, executes the instructions in order, and creates a new image based on those instructions. Here’s how docker build works:

1. Specify the Build Context: When you run docker build, you provide a directory path that serves as the build context. This is typically the directory containing the Dockerfile and any files that need to be included in the image. The build context is recursively sent to the Docker daemon, allowing it to access the necessary files and directories. For example, if your Dockerfile and application code are in a directory called “myapp,” you would navigate to that directory in the terminal and run docker build from there:

   cd /path/to/myapp
   docker build -t my-image .

In this example, the build context is set to the current directory (represented by .), which includes the Dockerfile and the application code.

2. Read and Parse the Dockerfile: Docker reads the Dockerfile located in the build context. The Dockerfile contains a series of instructions that define how the image should be constructed. These instructions specify things like the base image to use, the working directory, which files to copy into the image, how to install dependencies, and what command to run when the container starts.
3. Execute Build Steps: Docker executes each instruction in the Dockerfile in the order they are defined. These instructions can include:

• FROM: Specifies the base image to build upon. It’s the starting point for your image.
• WORKDIR: Sets the working directory inside the image where subsequent commands will be executed.
• COPY or ADD: Copies files and directories from the build context into the image.
• RUN: Executes shell commands within the image. Commonly used for installing packages, configuring software, and building application code.
• EXPOSE: Specifies which ports the container will listen on at runtime.
• ENV: Sets environment variables within the image.
• CMD or ENTRYPOINT: Defines the command that will be executed when a container is started from the image.

2. Create Image Layers: As Docker executes each instruction, it creates a new image layer. Docker images are composed of multiple layers, and each instruction in the Dockerfile results in a new layer. Layers are cached, and if an instruction doesn’t change (e.g., a package installation that’s already been performed), Docker can use the cached layer to speed up subsequent builds.
3. Tag and Name the Image: When the build process completes successfully, Docker tags the resulting image with a name and an optional version or tag. This allows you to easily reference and run containers based on this image in the future.
4. Clean Up: Docker cleans up any temporary files or artifacts generated during the build process.
5. Finalize the Image: The resulting image is a standalone, immutable, and portable package containing the application and its dependencies. You can use this image to create and run containers.

To summarize, docker build is the command used to create Docker images based on instructions provided in a Dockerfile. It allows you to automate the image creation process, ensuring consistency and repeatability in your container deployments.