server: buildfile: add NOCACHE instruction

This patch adds the `NOCACHE` instruction to the Dockerfile
instruction set. Basically, the NOCACHE instruction will run the
given instruction and arguments (and therefore any subsequent
instructions) without the cache enabled.

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