Use ubuntu package for dotnet.

.docker/base.Dockerfile

@@ -58,14 +58,7 @@ RUN { type -p curl >/dev/null || sudo apt-get install curl -y ; } \
     && sudo apt-get clean
 
 # CI dependencies: .NET Core
-# Repository install may incur some (transient?) failures (see for instance https://github.com/dotnet/sdk/issues/27082 )
-# So, we use manual install instead, from https://docs.microsoft.com/en-us/dotnet/core/install/linux-scripted-manual#manual-install
-ENV DOTNET_ROOT /home/opam/dotnet
-RUN wget -nv https://download.visualstudio.microsoft.com/download/pr/cd0d0a4d-2a6a-4d0d-b42e-dfd3b880e222/008a93f83aba6d1acf75ded3d2cfba24/dotnet-sdk-6.0.400-linux-x64.tar.gz && \
-    mkdir -p $DOTNET_ROOT && \
-    tar xf dotnet-sdk-6.0.400-linux-x64.tar.gz -C $DOTNET_ROOT && \
-    echo export PATH=$DOTNET_ROOT:'$PATH' | tee --append $HOME/.profile $HOME/.bashrc $HOME/.bash_profile && \
-    rm -f dotnet-sdk*.tar.gz
+RUN sudo apt-get install -y --no-install-recommends dotnet6 && sudo apt-get clean
 
 # Install OCaml
 ARG OCAML_VERSION=4.14.2