[BUILD] Ignore 'docker-clean' Output on Failure
[BUILD] Ignore 'docker-clean' Output on Failure

When docker images don't exist, or have already been removed, the
'docker-clean' target lists failures for 'docker rmi' because it
requires at least on argument.

This ensures at least one argument is passed ('' if no images).

Change-Id: Ie2a47ca028dbafd65fd643c0272bf64fc7bdd030
Signed-off-by: Trevor Bramwell <9318f446584e3f6cf68dea37a1a9fde63cb30ce1@linuxfoundation.org>

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