build dynamic MMG lib to that implicit linking with Scotch is ok

utils/docker/Dockerfile.ubuntu20.04

@@ -13,7 +13,7 @@ RUN apt-get update && apt-get install -y cargo && apt-get clean
 RUN cargo install --root /opt/cargo mdbook --version "^0.3.7"
 
 # mmg
-RUN git clone https://github.com/MmgTools/mmg.git && cd mmg && mkdir build && cd build && cmake .. && make -j8 && make install && cd .. && rm -rf mmg
+RUN git clone https://github.com/MmgTools/mmg.git && cd mmg && mkdir build && cd build && cmake -DBUILD_SHARED_LIBS=1 .. && make -j8 && make install && cd .. && rm -rf mmg
 
 # "docker build --build-arg REBUILD_GMSH=somethingnew"
 ARG REBUILD_GMSH=