[202012] Clear /etc/resolv.conf before building image (#12686)

Why I did it
nameserver and domain entries from build system fsroot gets into sonic image.

How I did it
Clear /etc/resolv.conf before building image

How to verify it
Built image with it and verified with install that /etc/resolv.conf is empty

Author: devpatha

build_debian.sh

@@ -590,6 +590,7 @@ sudo rm -f $ONIE_INSTALLER_PAYLOAD $FILESYSTEM_SQUASHFS
 sudo du -hsx $FILESYSTEM_ROOT
 sudo mkdir -p $FILESYSTEM_ROOT/var/lib/docker
 scripts/collect_host_image_version_files.sh $TARGET_PATH $FILESYSTEM_ROOT
+sudo cp files/image_config/resolv-config/resolv.conf $FILESYSTEM_ROOT/etc/resolv.conf
 sudo mksquashfs $FILESYSTEM_ROOT $FILESYSTEM_SQUASHFS -e boot -e var/lib/docker -e $PLATFORM_DIR
 
 # Ensure admin gid is 1000