General cleanup of the builder.md file

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
**Docker can act as a builder** by reading instructions from a
`Dockerfile`. The `Dockerfile` is a text file that contains the same instructions that you might
use to manually create an image. Executing `docker build` will run those
steps and commit each in order, giving you a final image.

or something like that?