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).
diff --git a/azure-pipelines/docker/bionic b/azure-pipelines/docker/bionic
index 65a1406..5918584 100644
--- a/azure-pipelines/docker/bionic
+++ b/azure-pipelines/docker/bionic
@@ -1,5 +1,4 @@
-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 \
diff --git a/azure-pipelines/docker/xenial b/azure-pipelines/docker/xenial
index bfb96d9..67867ed 100644
--- a/azure-pipelines/docker/xenial
+++ b/azure-pipelines/docker/xenial
@@ -1,5 +1,4 @@
-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 \