Explicitly stated that you must add --build-arg for each build argument.
Added multiple arguments to example of `--build-arg` usage.
Fix for https://github.com/docker/docker.github.io/issues/6248
Signed-off-by: Preston Cowley <cowlinator@gmail.com>
(cherry picked from commit ad44e2d45e)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>