Dockerfile: Add a comment about the manually installed CMake

Dockerfile.dev

@@ -7,6 +7,9 @@ RUN apt-get update -qq && apt-get install -qqy --no-install-recommends \
     apt-get clean -y && \
     rm -rf /var/lib/apt/lists/*
 
+# Manually install a newer version of CMake; this is needed since building
+# LLVM requires CMake 3.13.4, while Ubuntu 18.04 ships with 3.10.2. If
+# updating to a newer distribution, this can be dropped.
 RUN cd /opt && \
     wget https://github.com/Kitware/CMake/releases/download/v3.20.1/cmake-3.20.1-Linux-$(uname -m).tar.gz && \
     tar -zxvf cmake-*.tar.gz && \