Improved the "test" target in the Makefile file to make the cleaning of the docker image optional.

Makefile

@@ -50,10 +50,12 @@ ifeq ($(docker),yes)
 		--workdir ${DOCKER_VOLUME_TARGET} \
 		--platform ${platform} \
 		${DOCKER_IMAGE_NAME}:${tag} \
-		swift test --parallel
+		swift test --configuration ${config}
+    ifeq ($(clean),yes)
 		@docker rmi ${DOCKER_IMAGE_NAME}:${tag}
+    endif
 else
-	@swift test --parallel
+	@swift test --configuration ${config}
 endif
 
 # --- HOUSE-KEEPING ---