Dockerfile.cross: Download a newer version of cmake than what the base distribution has

When building lldb for windows with the llvm 10.0.0 version,
cmake 3.13 is required. (Ubuntu 18.04 comes with cmake 3.10.2.)

Author: mstorsjo

Dockerfile.cross

@@ -16,6 +16,13 @@ RUN apt-get update -qq && \
     apt-get clean -y && \
     rm -rf /var/lib/apt/lists/*
 
+RUN cd /opt && \
+    wget https://github.com/Kitware/CMake/releases/download/v3.16.2/cmake-3.16.2-Linux-x86_64.tar.gz && \
+    tar -zxvf cmake-*.tar.gz && \
+    rm cmake-*.tar.gz && \
+    mv cmake-* cmake
+ENV PATH=/opt/cmake/bin:$PATH
+
 ARG CROSS_ARCH=x86_64
 ENV CROSS_TOOLCHAIN_PREFIX=/opt/llvm-mingw-$CROSS_ARCH