rpb-ltd-gateway-image: force docker as part of the core image
rpb-ltd-gateway-image: force docker as part of the core image

Even if not enabled/supported by the RPB layer, force docker by default
into the rpb-ltd-gateway-image.

Signed-off-by: Ricardo Salveti <6fd42f733b6586c555e01d4ff3116fc7e73e7330@linaro.org>

linaro-technologies/meta-ltd