chore(dockerfile): reduce packaging time

The packages aren't updated frequently, so the content of the image
layer usually remains unchanged.
To avoid unnecessary rebuilding of this layer, copy the longhorn-manager
binaries after installing the packages.

Longhorn 8744

Signed-off-by: Derek Su <derek.su@suse.com>
(cherry picked from commit 201e11a)

package/Dockerfile

@@ -13,13 +13,15 @@ RUN if [ "$TARGETPLATFORM" != "linux/amd64" ] && [ "$TARGETPLATFORM" != "linux/a
 
 ENV ARCH ${TARGETPLATFORM#linux/}
 
-COPY package/bin/longhorn-manager-${ARCH} /usr/local/sbin/longhorn-manager
-
-COPY package/launch-manager package/nsmounter /usr/local/sbin/
 
 RUN zypper -n install iputils iproute2 nfs-client cifs-utils bind-utils e2fsprogs xfsprogs zip unzip && \
     rm -rf /var/cache/zypp/*
 
 VOLUME /usr/local/sbin
+
+COPY package/bin/longhorn-manager-${ARCH} /usr/local/sbin/longhorn-manager
+
+COPY package/launch-manager package/nsmounter /usr/local/sbin/
+
 EXPOSE 9500
 CMD ["launch-manager"]