ADD: added new version of protobuf

libs/protobuf/kokoro/linux/dockerfile/test/csharp/Dockerfile

@@ -0,0 +1,41 @@
+FROM debian:buster
+
+# Install dependencies.  We start with the basic ones require to build protoc
+# and the C++ build
+RUN apt-get update && apt-get install -y \
+  autoconf \
+  autotools-dev \
+  build-essential \
+  bzip2 \
+  ccache \
+  curl \
+  gcc \
+  git \
+  libc6 \
+  libc6-dbg \
+  libc6-dev \
+  libgtest-dev \
+  libtool \
+  make \
+  parallel \
+  time \
+  wget \
+  && apt-get clean
+
+# Update ca-certificates to fix known buster + .NET 5 issue
+# https://github.com/NuGet/Announcements/issues/49
+RUN apt-get update && apt-get install -y ca-certificates && apt-get clean
+
+# dotnet SDK prerequisites
+RUN apt-get update && apt-get install -y libunwind8 libicu63 && apt-get clean
+
+# Install dotnet SDK via install script
+RUN wget -q https://dot.net/v1/dotnet-install.sh && \
+    chmod u+x dotnet-install.sh && \
+    ./dotnet-install.sh --version 3.1.415 && \
+    ./dotnet-install.sh --version 6.0.100 && \
+    ln -s /root/.dotnet/dotnet /usr/local/bin
+
+RUN wget -q www.nuget.org/NuGet.exe -O /usr/local/bin/nuget.exe
+
+ENV DOTNET_SKIP_FIRST_TIME_EXPERIENCE true