docker: Install CMake 3.12 in Ubuntu 18.04 image.

CMake 3.10 in Ubuntu 18.04 has a bug where it finds Python 2 even when
Python 3 is present.  Install 3.12, where the bug is fixed.

* docker/Dockerfile_ubuntu_18.04: Download, check and extract CMake
3.12 to /usr/local.

Change-Id: I8e344e630cfcdc58d8e3579ac0b7c22314264ab9

docker/Dockerfile_ubuntu_18.04

@@ -11,6 +11,12 @@ RUN apt-get update && \
         g++-8 \
         wget
 
+# FindPythonInterp in CMake < 3.12 is buggy; install 3.12.
+RUN wget https://cmake.org/files/v3.12/cmake-3.12.0-Linux-x86_64.tar.gz && \
+    echo "2cc891a77f5850ffecb2d358f3a84cd9b90e6941496ca531ea31e7bd7600a373  \
+cmake-3.12.0-Linux-x86_64.tar.gz" | sha256sum --check && \
+    tar xf cmake-3.12.0-Linux-x86_64.tar.gz --strip-components=1 -C /usr/local
+
 ADD scripts/prebuild-package-debian.sh /opt/prebuild-package-debian.sh
 
 COPY packaging/rules/debian-qt/control /tmp/builddeps/debian/control