Let's see what happens if we add clang 11 and 12 and bump cmake

Dockerfile-ubuntu2004

     apt-get install -y clang-10 lldb-10 lld-10 && \
     apt-get install -y libc++-10-dev libc++-10-dev clang-format-10 && \
     \
+    echo deb http://apt.llvm.org/focal/ llvm-toolchain-focal-11 main >> /etc/apt/sources.list && \
+    echo deb-src http://apt.llvm.org/focal/ llvm-toolchain-focal-11 main >> /etc/apt/sources.list && \
+    wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key|sudo apt-key add - ; \
+    apt update -y && \
+    apt-get install -y clang-11 lldb-11 lld-11 && \
+    apt-get install -y libc++-11-dev libc++-11-dev clang-format-11 && \
+    \
+    echo deb http://apt.llvm.org/focal/ llvm-toolchain-focal-12 main >> /etc/apt/sources.list && \
+    echo deb-src http://apt.llvm.org/focal/ llvm-toolchain-focal-12 main >> /etc/apt/sources.list && \
+    wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key|sudo apt-key add - ; \
+    apt update -y && \
+    apt-get install -y clang-12 lldb-12 lld-12 && \
+    apt-get install -y libc++-12-dev libc++-12-dev clang-format-12 && \
+    \
     rm -rf /var/lib/apt/lists/*
 
 # Install newer CMake to avoid some warnings the "FindBoost" script throws on
 #  more recent versions of boost that the system CMake doesn't now about.
 # I chose a bloody new boost, so I need to use the latest RC1 (at the time of writing)
-RUN wget https://github.com/Kitware/CMake/releases/download/v3.18.0-rc1/cmake-3.18.0-rc1.tar.gz && \
-    tar -zxf cmake-3.18.0-rc1.tar.gz && \
-    cd cmake-3.18.0-rc1 && \
+RUN wget https://github.com/Kitware/CMake/releases/download/v3.21.3/cmake-3.21.3.tar.gz && \
+    tar -zxf cmake-3.21.3.tar.gz && \
+    cd cmake-3.21.3 && \
     ./bootstrap && \
     make -j $(nproc) && \
     make install && \
     cd .. && \
-    rm -rf cmake-3.18.0-rc1 && \
-    rm -rf cmake-3.18.0-rc1.tar.gz
+    rm -rf cmake-3.21.3 && \
+    rm -rf cmake-3.21.3.tar.gz
 
 # Fix some weird linker issue CAF build runs into. (fails to link -lc++abi)
 RUN cp -prv /usr/lib/llvm-10/lib/libc++abi.so.1.0 /usr/lib/llvm-10/lib/libc++abi.so
 
+RUN cp -prv /usr/lib/llvm-11/lib/libc++abi.so.1.0 /usr/lib/llvm-11/lib/libc++abi.so
+
+RUN cp -prv /usr/lib/llvm-12/lib/libc++abi.so.1.0 /usr/lib/llvm-12/lib/libc++abi.so
+
 CMD "/bin/bash"