azure: docker: synchronize Xenial/Bionic build instructions

Our two Docker build instructions for Xenial and Bionic have diverged a
bit. Let's re-synchronize them with each other to make them as similar
as possible.

azure-pipelines/docker/bionic

 ARG BASE
-FROM $BASE
+FROM $BASE AS apt
 RUN apt-get update && \
-    apt-get install -y --no-install-recommends \
+    DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
         clang \
         cmake \
         curl \
@@ -22,8 +22,8 @@ RUN apt-get update && \
         valgrind \
         && \
     rm -rf /var/lib/apt/lists/*
-RUN mkdir /var/run/sshd
 
+FROM apt AS mbedtls
 RUN cd /tmp && \
     curl --location --silent https://tls.mbed.org/download/mbedtls-2.16.2-apache.tgz | \
     tar -xz && \
@@ -34,7 +34,9 @@ RUN cd /tmp && \
     cd .. && \
     rm -rf mbedtls-2.16.2
 
+FROM mbedtls AS configure
 COPY entrypoint.sh /usr/local/bin/entrypoint.sh
 RUN chmod a+x /usr/local/bin/entrypoint.sh
+RUN mkdir /var/run/sshd
 
 ENTRYPOINT ["/usr/local/bin/entrypoint.sh"]

azure-pipelines/docker/xenial

@@ -23,7 +23,9 @@ RUN apt-get update && \
         openssl \
         pkgconf \
         python \
-        valgrind
+        valgrind \
+        && \
+    rm -rf /var/lib/apt/lists/*
 
 FROM apt AS mbedtls
 RUN cd /tmp && \