chore: add build date

dist/images/Dockerfile

 # syntax = docker/dockerfile:experimental
 FROM centos:8 as ovs-rpm
-
+ENV BUILD_DATE 20200731
 ARG RPM_ARCH
 ARG ARCH
 
@@ -20,6 +20,7 @@ RUN bash /root/build.sh && \
 
 
 FROM centos:8
+ENV BUILD_DATE 20200731
 RUN yum remove -y bind-export-libs && yum update -y && \
     yum install -y \
         firewalld-filesystem \

dist/images/Dockerfile.dpdk1911

 # NOTE: At time of writing, CentOS 8 does not have dpdk-devel v19.11 in the Yum repo.
 # This package is required to build the OVS-DPDK RPMs and is available in Fedora 32.
 FROM fedora:32 as dpdk
-
+ENV BUILD_DATE 20200731
 ENV RPM_ARCH=x86_64
 
 ENV DPDK_VERSION=19.11.1