Dennis added a 'subvariant' property to the Docker image config,
but we weren't using it, we were just hard-coding the variant
as the subvariant...and doing it wrong, as well, using the
variant object rather than its uid property.
This fixes both problems: we'll now use the explicitly set
image config 'subvariant' property if there is one, otherwise
we use the variant.uid as the subvariant.
Note this is a high priority fix, as it broke F24 compose today.
Signed-off-by: Adam Williamson <awilliam@redhat.com>
c321bca8ef