Use Docker BuildKit for the CI and docker_script

Docker BuildKit has a number of advanages over the tradiditonal
Docker back end - it's potentially quicker as it can make use of
parallel jobs when multi-stage Dockerfiles are used, and it also
enables the use of automatically set target/host ARGs, such as
TARGETARCH. This is especially useful when working with multiple
target architectures.

scripts/docker_build

@@ -12,6 +12,9 @@ source "$SCRIPTS_DIR/common"
 readonly DOCKER_UID="${UID:-0}"
 readonly DOCKER_GID="$(id -g)"
 readonly DOCKER_USER="${USER:-root}"
+# Use the Docker BuildKit backend: Useful for quicker builds, and multi-arch 
+# builds, as it expands the automatic variables available to the container.
+declare -x DOCKER_BUILDKIT=1
 
 docker build \
   --cache-from="$DOCKER_IMAGE_NAME:latest" \