server: buildfile: ensure that ONBUILD triggers aren't committed

This patch fixes the bug where ONBUILD triggers are committed each build
step created during the ONBUILD trigger execution, since the triggers are
only wiped *after* all ONBUILD trigger steps have been committed. This was
fixed by simply copying the ONBUILD triggers and wiping the config
*before* committing anything.

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

This patch adds CLI integration tests to ensure that NOCACHE instructions
in Dockerfiles only apply to direct children of the original image.

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)
Merge pull request #6451 from cyphar/6445-fix-onbuild-passing-to-grandchildren

Ensure that ONBUILD triggers aren't committed to grandchildren