How is an image built from a Dockerfile?

An image is built from a Dockerfile with the docker build command, where Docker reads the file line by line and, at each step, forms a new layer of the future image.

#The process, junior-style

1. You write a Dockerfile - a set of instructions (FROM, COPY, RUN, CMD, and so on).
2. You run the image build:

3. Docker reads the Dockerfile top to bottom and:

• takes the base image (FROM)
• copies files (COPY)
• runs commands (RUN)
• sets up the environment

4. Each instruction becomes a layer of the future image.
5. After executing all the instructions, Docker saves the image under the given name and tag (myimage).

#What is important to understand

• Docker caches layers - if a line has not changed, the layer is not rebuilt.

• The build result is an image, which you can view with:

  bash

  Copy

  docker images

#Summary

Docker builds an image by executing the Dockerfile instructions step by step during docker build, forming a layered template of a container ready to run.