Dockerfile is an instruction set to set up a new container. It looks like a BASH script that serially runs all the mentioned commands. The commands are predefined by Dockerfile syntax.

Unlike BASH script, Dockerfile runs and applies effects of a command to the output of the previous step. Each step of a Dockerfile creates, by default, a container which is kept hidden. You can list such ephemeral containers by running following command:

$ docker images -a 

All containers with <none> name are ephemeral.

Why Docker need Ephemeral Containers

• Each ephemeral container acts as a cached output of a step in the Dockerfile.
• Next container build would use the cached output instead of running the step again.

How it Works?

• Each step starting from the from base checks if the next step has cached output.
• The check is with the asked instruction and the instruction that was run by the cached output.
• If instructions do not match, the cache is invalidated. The step is built normally.
• To disable caching, provide the no-cache option.

  $ docker build --no-cache ...
  

Reference

• https://takacsmark.com/dockerfile-tutorial-by-example-dockerfile-best-practices-2018/
• https://docs.docker.com/develop/develop-images/dockerfile_best-practices/