message: Fix linux Dockerfile to include Doxygen
diff: FROM mcr.microsoft.com/dotnet/core/sdk:3.1-focal
  -RUN apt-get update
  -RUN apt-get install -y clang cmake g++ python3
  -RUN apt-get install -y libfreetype-dev libglfw3-dev libglm-dev libgtest-dev \
  - libgmock-dev libsndfile1-dev liblua5.3-dev libopenal-dev libspdlog-dev
  -RUN apt-get install -y python3-glad
  -RUN rm -rf /var/lib/apt/lists/*
  +RUN apt-get update \
  + && apt-get install -y \
  + clang \
  + cmake \
  + doxygen \
  + graphviz \
  + g++ \
  + libfreetype-dev \
  + libglfw3-dev \
  + libglm-dev \
  + libgmock-dev \
  + libgtest-dev \
  + liblua5.3-dev \
  + libopenal-dev \
  + libsndfile1-dev \
  + libspdlog-dev \
  + python3-glad \
  + && rm -rf /var/lib/apt/lists/*
  