Update build.md
Update build.md

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 <7b489479b3bdcd9d6cdb2cc52eda253707af104b@gmail.com>

cpuguy83/cli,thaJeztah/cli,docker/cli,cpuguy83/cli,thaJeztah/cli,docker/cli