Install CMake in Merlin base image (instead of copying from build)

Since the CMake install is a bit more involved than just copying the executable, let's just install it in the base container directly.

docker/dockerfile.merlin

@@ -264,8 +264,9 @@ RUN apt update -y --fix-missing && \
 
 RUN ln -s /usr/bin/python3 /usr/bin/python
 
-# CMake
-COPY --chown=1000:1000 --from=build /usr/local/bin/cmake /usr/local/bin/cmake
+# Install cmake (used by downstream images)
+RUN apt remove --purge cmake -y && wget http://www.cmake.org/files/v3.21/cmake-3.21.1.tar.gz && \
+    tar xf cmake-3.21.1.tar.gz && cd cmake-3.21.1 && ./configure && make -j$(nproc) && make install
 
 # Includes
 COPY --chown=1000:1000 --from=build /usr/include /usr/include/