ci: update container versions

Update all the container versions to force a rebuild so that they'll get
tagged with latest (due to changes in the CI scripts).

ci/docker/bionic

 ARG BASE=ubuntu:bionic
+
 FROM ${BASE} AS apt
 RUN apt-get update && \
     DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \

ci/docker/centos7

 ARG BASE=centos:7
+
 FROM ${BASE} AS yum
 RUN yum install -y \
 	which \

ci/docker/centos8

 ARG BASE=centos:8
+
 FROM ${BASE} AS yum
 RUN yum install -y \
 	which \

ci/docker/docurium

 ARG BASE=ubuntu:bionic
+
 FROM ${BASE}
 RUN apt update && apt install -y cmake pkg-config ruby ruby-dev llvm libclang-dev libssl-dev python-pygments
 RUN gem install docurium

ci/docker/focal

 ARG BASE=ubuntu:focal
+
 FROM ${BASE} AS apt
 RUN apt-get update && \
     DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \

ci/docker/xenial

 ARG BASE=ubuntu:xenial
+
 FROM ${BASE} AS apt
 RUN apt-get update && \
     DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \