Enable multi-platform image for CI

Multi-platform images simplifies usage as there is no need to
use dedicated tag for different platform. But a special care need
to be taken when building the image. Current version expect native,
or emulated, support from builder.

Signed-off-by: Anton Bondarenko <anton.bondarenko@arm.com>

Approved-by: Jakub Sujak <jakub.sujak@arm.com>