docker: install mbedTLS on both Bionic and Xenial

We're about to phase out support for Trusty, but neither Bionic nor
Xenial images provide the mbedTLS library that's available in Trusty.
Build them for both to pull them in line with Trusty.

azure-pipelines/docker/bionic

@@ -5,3 +5,15 @@ RUN apt-get update
 RUN apt-get -y install pkgconf clang git cmake curl libssl-dev libcurl4 libcurl4-openssl-dev libssh2-1-dev libz-dev valgrind openssh-client openssh-server
 RUN if [ "$ARCH" != "armhf" -a "$ARCH" != "arm64" ]; then apt-get -y install openjdk-11-jre-headless; fi
 RUN mkdir /var/run/sshd
+
+RUN cd /tmp && \
+    curl -LO https://tls.mbed.org/download/mbedtls-2.16.2-apache.tgz && \
+    tar -xf mbedtls-2.16.2-apache.tgz && \
+    rm -f mbedtls-2.16.2-apache.tgz && \
+    cd mbedtls-2.16.2 && \
+    scripts/config.pl set MBEDTLS_MD4_C 1 && \
+    CFLAGS=-fPIC cmake -DENABLE_PROGRAMS=OFF -DENABLE_TESTING=OFF -DUSE_SHARED_MBEDTLS_LIBRARY=OFF -DUSE_STATIC_MBEDTLS_LIBRARY=ON . && \
+    cmake --build . && \
+    make install && \
+    cd .. && \
+    rm -rf mbedtls-2.16.2

azure-pipelines/docker/xenial

@@ -4,3 +4,15 @@ ARG CACHEBUST=1
 RUN apt-get update
 RUN apt-get -y install pkgconf clang git cmake curl libssl-dev libcurl3 libcurl3-gnutls libcurl4-gnutls-dev valgrind openssh-client openssh-server openjdk-8-jre
 RUN mkdir /var/run/sshd
+
+RUN cd /tmp && \
+    curl -LO https://tls.mbed.org/download/mbedtls-2.16.2-apache.tgz && \
+    tar -xf mbedtls-2.16.2-apache.tgz && \
+    rm -f mbedtls-2.16.2-apache.tgz && \
+    cd mbedtls-2.16.2 && \
+    scripts/config.pl set MBEDTLS_MD4_C 1 && \
+    CFLAGS=-fPIC cmake -DENABLE_PROGRAMS=OFF -DENABLE_TESTING=OFF -DUSE_SHARED_MBEDTLS_LIBRARY=OFF -DUSE_STATIC_MBEDTLS_LIBRARY=ON . && \
+    cmake --build . && \
+    make install && \
+    cd .. && \
+    rm -rf mbedtls-2.16.2