Update Dockerfile.dapper

Signed-off-by: David Ko <innobead@outlook.com>

Dockerfile.dapper

@@ -140,7 +140,7 @@ RUN cd /go/src/github.com/longhorn && \
     cd longhorn-instance-manager && \
     go build -o ./longhorn-instance-manager && \
     cp -r integration/rpc/ ${DAPPER_SOURCE}/integration/rpc/ && \
-    cp longhorn-instance-manager /usr/local/bin
+    install longhorn-instance-manager /usr/local/bin
 
 VOLUME /tmp
 ENV TMPDIR /tmp