docs: sources: reference: builder: add NOCACHE instruction docs

This patch adds the relevant reference documentation for the
new `NOCACHE` instruction.

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)
integration-cli: *: added test for NOCACHE

This patch adds integration-cli tests for the `NOCACHE` instruction,
ensuring that the instruction really isn't cached by docker.

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)