Docs for Official Repo release

Changes and additions for the Official Repo release on Wednesday.
`You should use `WORKDIR` instead of instructions like `RUN cd … && do-something`, which are hard to read,
troubleshoot, and maintain.`

So we're suggesting that If I have to run 10 `RUN cd /differentdir && make install` I should instead have 20 layers?

Sorry, this one feels hard to justify.