This commit adds the image variant to the image.(Image) type and
updates related functionality. Images built from another will
inherit the OS, architecture and variant.
Note that if a base image does not specify an architecture, the
local machine's architecture is used for inherited images. On the
other hand, the variant is set equal to the parent image's variant,
even when the parent image's variant is unset.
The legacy builder is also updated to allow the user to specify
a '--platform' argument on the command line when creating an image
FROM scratch. A complete platform specification, including variant,
is supported. The built image will include the variant, as will any
derived images.
Signed-off-by: Chris Price <chris.price@docker.com>
