Add C# compilation with Mono (algorithm-archivists#1013)

Dockerfile

@@ -6,7 +6,7 @@ FROM mcr.microsoft.com/vscode/devcontainers/base:0-${VARIANT}
 
 # [Optional] Uncomment this section to install additional OS packages.
 RUN apt-get update && export DEBIAN_FRONTEND=noninteractive \
-    && apt-get -y install --no-install-recommends build-essential software-properties-common xz-utils g++ sbcl julia python3 python3-pip python3-dev ghc openjdk-11-jdk libssl-dev gfortran libxml2-dev libyaml-dev libgmp-dev libz-dev libncurses5 gnuplot nodejs npm lua5.3 ocaml php ruby-full gnu-smalltalk scratch libfftw3-dev cmake
+    && apt-get -y install --no-install-recommends build-essential software-properties-common xz-utils g++ sbcl julia python3 python3-pip python3-dev ghc openjdk-11-jdk libssl-dev gfortran libxml2-dev libyaml-dev libgmp-dev libz-dev libncurses5 gnuplot nodejs npm lua5.3 ocaml php ruby-full gnu-smalltalk scratch libfftw3-dev cmake mono-devel
 
 # Setup Crystal
 RUN echo 'deb http://download.opensuse.org/repositories/devel:/languages:/crystal/xUbuntu_20.04/ /' | sudo tee /etc/apt/sources.list.d/devel:languages:crystal.list
@@ -98,7 +98,7 @@ ENV PATH=$PATH:~/elm
 
 # Setup V
 RUN mkdir -p ~/vlang && wget https://github.com/vlang/v/releases/download/weekly.2021.44/v_linux.zip -O ~/vlang/vlang.zip && \
-    unzip ~/vlang/vlang.zip -d ~/vlang 
+    unzip ~/vlang/vlang.zip -d ~/vlang
 ENV PATH=$PATH:~/vlang/v
 
 # Install the packages that needed extra help