What is the purpose of Dockerfile’s ONBUILD instruction and when is it useful?

Question

Docker Fundamentals — Hard

What is the purpose of Dockerfile’s ONBUILD instruction and when is it useful?

Accepted Answer

The ONBUILD instruction in a Dockerfile registers a trigger that executes when the image is used as a base image in another Dockerfile's FROM instruction. This is useful for automating setup steps in child images without repeating them.