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A Dockerfile is a text file that contains a sequence of instructions used to build a Docker image.

When you run docker build, Docker reads this file step by step and creates an image layer by layer.

This guide explains:

• What happens internally during a build
• The most important Dockerfile instructions
• The difference between build-time and runtime concepts
• How layers and caching work

1. What Happens When You Run docker build

When you execute:

docker build -t myapp .

Docker performs these steps:

1. Reads the Dockerfile.
2. Sends the build context (the current directory) to the Docker daemon.
3. Executes each instruction one by one.
4. Creates a new image layer for most instructions.
5. Caches each layer for future builds.
6. Produces a final image.