[build]: create empty /var/lib/docker if needed (#1920)

stretch docker-engine in base image is not started by default
in the build process. Need to create empty /var/lib/docker

Signed-off-by: Guohan Lu <gulv@microsoft.com>

build_debian.sh

@@ -396,6 +396,7 @@ sudo rm -f $ONIE_INSTALLER_PAYLOAD $FILESYSTEM_SQUASHFS
 ## Output the file system total size for diag purpose
 ## Note: -x to skip directories on different file systems, such as /proc
 sudo du -hsx $FILESYSTEM_ROOT
+sudo mkdir -p $FILESYSTEM_ROOT/var/lib/docker
 sudo mksquashfs $FILESYSTEM_ROOT $FILESYSTEM_SQUASHFS -e boot -e var/lib/docker -e $PLATFORM_DIR
 
 ## Compress docker files