[Build] Fix the installation candidate not found issue when building docker-sonic-vs (#14439)

Why I did it
Fix the installation candidate not found issue when building docker-sonic-vs

How I did it
Need to run the command "apt-get update" to update the mirror indexes before installing the package gnupg

How to verify it

Author: xumia

platform/vs/docker-sonic-vs/Dockerfile.j2

@@ -8,7 +8,7 @@ RUN [ -f /etc/rsyslog.conf ] && sed -ri "s/%syslogtag%/$docker_container_name#%s
 ## Make apt-get non-interactive
 ENV DEBIAN_FRONTEND=noninteractive
 
-RUN apt-get install -y gnupg
+RUN apt-get update && apt-get install -y gnupg
 COPY ["sonic-dev.gpg.key", "/etc/apt/"]
 RUN apt-key add /etc/apt/sonic-dev.gpg.key
 RUN echo "deb http://packages.microsoft.com/repos/sonic-dev/ jessie main" >> /etc/apt/sources.list