Dockerfile add NOCACHE instruction to specification

This patchset adds support for the `NOCACHE` instruction (and the relevant docs). Basically, the `NOCACHE` instruction tags a certain instruction as being "uncachable", and causes the build to stop using the cache from that instruction onward. This patchset implements #1996.

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)

Closes #1996.

/cc @shykes 
/cc @jamtur01 @SvenDowideit @OSTezer
Whitespace. Sorry, my editor automatically strips trailing whitespace from lines on saving the file. Do you want me to remove it from the commit?