build: fix image build from make build

he introduction of the target for the creation of the tools
directory to common.mk changed make build to not build container
images any more because the tools cache dir target masked the default
images building goal  accidentally by including common.mk before target
definitions.

This change re-establishes desired behavior of make build
by explicitly specifying the default goal for image building.

Signed-off-by: Michael Adam <obnox@samba.org>

images/image.mk

@@ -21,6 +21,12 @@ override GOOS=linux
 SELF_DIR := $(dir $(lastword $(MAKEFILE_LIST)))
 include $(SELF_DIR)/../build/makelib/common.mk
 
+# common.mk may define targets (e.g., directory creation). Since GNU Make uses the first target
+# it sees as the default goal, this behavior may unintentionally mask the intended default goal
+# of all (AKA build).
+# Explicitly set the default goal for image builds to override this masking.
+.DEFAULT_GOAL := all
+
 # the registry used for cached images
 CACHE_REGISTRY := cache