docker: don't take BASE as an argument

The xenial image depends on ubuntu:xenial; the bionic one on
ubuntu:bionic.  No need for this to be a variable, that's just
additional (unnecessary) state to manage in the CI setup(s).

azure-pipelines/docker/bionic

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

azure-pipelines/docker/xenial

-ARG BASE
-FROM $BASE AS apt
+FROM ubuntu:xenial AS apt
 RUN apt-get update && \
     DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
         bzip2 \