fix: copy synocli into builder image for default make target

The Makefile default build target includes bin/synocli, but the
Dockerfile builder stage never copies synocli/ into the build context,
so building the image fails with
"stat /go/src/synok8scsiplugin/synocli: directory not found".

Copy synocli/ so the in-image make behaves the same as in a full
checkout.

Dockerfile

@@ -15,6 +15,7 @@ ARG TARGETPLATFORM
 
 COPY main.go .
 COPY pkg ./pkg
+COPY synocli ./synocli
 RUN env GOARCH=$(echo "$TARGETPLATFORM" | cut -f2 -d/) \
         GOARM=$(echo "$TARGETPLATFORM" | cut -f3 -d/ | cut -c2-) \
         make