Also fixed some examples of using `docker build` to clarify that the
positional argument is a directory, not a file.
Also fixed some terminology. Dockerfiles contain instructions, not directives or
commands.
Signed-off-by: Daniel Nephin <dnephin@docker.com>
Upstream-commit: 3437715e20b3a1b16ea93a47802794b58b007863
Component: engine