Add ENV variables support to WORKDIR build command

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Docker-DCO-1.1-Signed-off-by: Adrien Folie <folie.adrien@gmail.com> (github: folieadrien)

Conflicts:
	builder/builder.go
		This file has been deleted.
Merge pull request #8341 from unclejack/add_workdir_test

add test for workdir env vars and add docs