Copy or mount from stage, image, or context
As of Dockerfile syntax 1.11, you can useONBUILDwith instructions that copy
or mount files from other stages, images, or build contexts. For example:
If the source offromis a build stage, the stage must be defined in the
Dockerfile whereONBUILDgets triggered. If it's a named context, that
context must be passed to the downstream build.