Update CMake to 3.18.0-rc1 to get rid of annoying cmake warning about boost 0.17.3

Dockerfile-ubuntu1804

@@ -18,15 +18,16 @@ RUN apt-get update && \
 
 # 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.
-RUN wget https://github.com/Kitware/CMake/releases/download/v3.16.8/cmake-3.16.8.tar.gz && \
-    tar -zxf cmake-3.16.8.tar.gz && \
-    cd cmake-3.16.8 && \
+# 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 && \
     ./bootstrap && \
     make -j $(nproc) && \
     make install && \
     cd .. && \
-    rm -rf cmake-3.16.8 && \
-    rm -rf cmake-3.16.8.tar.gz
+    rm -rf cmake-3.18.0-rc1 && \
+    rm -rf cmake-3.18.0-rc1.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