docker: install libssh2 1.8.2 on Xenial

While Xenial provides libssh2 in its repositories, it only has version
1.5.0 available. This version will unfortunately not be able to connect
to GitHub due to their removal of weak cryptographic standards [1]. To
still enable our CI to execute tests against GitHub, we thus have to
update the provided libssh2 version to a newer one.

Manually install libssh2 1.8.2 on Xenial. There's no need to do the same
for Bionic, as it already provides libssh2 1.8.0.

[1]: https://github.blog/2018-02-01-crypto-removal-notice/

azure-pipelines/docker/xenial

@@ -16,3 +16,13 @@ RUN cd /tmp && \
     make install && \
     cd .. && \
     rm -rf mbedtls-2.16.2
+
+RUN cd /tmp && \
+    curl -LO https://www.libssh2.org/download/libssh2-1.8.2.tar.gz && \
+    tar -xf libssh2-1.8.2.tar.gz && \
+    rm -f libssh2-1.8.2.tar.gz && \
+    cd libssh2-1.8.2 && \
+    CFLAGS=-fPIC cmake -G Ninja -DCRYPTO_BACKEND=Libgcrypt . && \
+    ninja install && \
+    cd .. && \
+    rm -rf libssh2-1.8.2