Add building for docker in weed/Makefile

Building without `CGO_ENABLED=0` and using the executable in docker can result in a docker container exiting with an error

weed/Makefile

@@ -9,6 +9,9 @@ all: install
 install:
 	go install -ldflags="-s -w"
 
+build_docker:
+	CGO_ENABLED=0 go build -ldflags="-s -w"
+
 clean:
 	go clean $(SOURCE_DIR)
 	rm -f $(BINARY)