docker: install cmake from binary, xenials is too old to build llvm

Dockerfile

@@ -6,11 +6,19 @@ FROM ubuntu:xenial
 
 RUN apt-get update \
  && apt-get install -y --no-install-recommends \
+        curl \
+        ca-certificates \
         build-essential \
         clang \
-        cmake \
-        python \
+        python3 \
         git \
         ninja-build \
  && apt-get clean \
  && rm -rf /var/lib/apt/lists/*
+
+RUN curl -sSLO https://github.com/Kitware/CMake/releases/download/v3.20.1/cmake-3.20.1-linux-x86_64.tar.gz \
+  && tar xf cmake-3.20.1-linux-x86_64.tar.gz \
+  && rm cmake-3.20.1-linux-x86_64.tar.gz \
+  && mkdir -p /opt \
+  && mv cmake-3.20.1-linux-x86_64 /opt/cmake
+ENV PATH /opt/cmake/bin:$PATH