General cleanup of the builder.md file

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
really? almost no-one uses the exec form of RUN - its justified for ENTRYPOINT and CMD, but I don't think it makes much sense for RUN - iirc, there are consequences like variable substitution that make this not the mainstream choice.