While checkpatch.pl can validate DCO sign off that job must always be
advisory only since it is expected that certain patches will fail some
code style rules.

We require the DCO sign off to be mandatory for all commits though, so
it benefits from being validated in a standalone job.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200918132903.1848939-3-berrange@redhat.com>
[thuth: Use "stage: build" to let it run earlier]
Signed-off-by: Thomas Huth <thuth@redhat.com>