Use latest stable mono instead of a hardcoded version (#28467)

build/scripts/docker/Dockerfile

@@ -33,10 +33,10 @@ RUN apt-get install -y  libunwind8 \
 
 # Install Mono
 RUN apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF && \
-    (echo "deb http://download.mono-project.com/repo/ubuntu stable-xenial/snapshots/5.8.0.108 main" | \
+    (echo "deb http://download.mono-project.com/repo/ubuntu stable-xenial main" | \
     tee /etc/apt/sources.list.d/mono-official.list) && \
     apt-get update && \
-    apt-get install -y mono-devel=5.8.0.108-0xamarin1+ubuntu1604b1 && \
+    apt-get install -y mono-devel && \
     apt-get clean
 
 # Setup User to match Host User, and give superuser permissions