ran script to update actual docker files

src/csharp/Grpc.Core/project.json

@@ -31,7 +31,7 @@
     "xmlDoc": true
   },
   "dependencies": {
-    "System.Interactive.Async": "3.0.0"
+    "Ix-Async": "1.2.5"
   },
   "frameworks": {
     "net45": { },

tools/dockerfile/interoptest/grpc_interop_csharp/Dockerfile

@@ -80,6 +80,8 @@ RUN apt-get update && apt-get -y dist-upgrade && apt-get install -y \
     nuget \
     && apt-get clean
 
+RUN nuget update -self
+
 # Prepare ccache
 RUN ln -s /usr/bin/ccache /usr/local/bin/gcc
 RUN ln -s /usr/bin/ccache /usr/local/bin/g++

tools/dockerfile/stress_test/grpc_interop_stress_csharp/Dockerfile

@@ -97,5 +97,7 @@ RUN apt-get update && apt-get -y dist-upgrade && apt-get install -y \
     nuget \
     && apt-get clean
 
+RUN nuget update -self
+
 # Define the default command.
 CMD ["bash"]

tools/dockerfile/test/csharp_coreclr_x64/Dockerfile

@@ -80,6 +80,8 @@ RUN apt-get update && apt-get -y dist-upgrade && apt-get install -y \
     nuget \
     && apt-get clean
 
+RUN nuget update -self
+
 
 # Install dotnet SDK based on https://www.microsoft.com/net/core#debian
 RUN apt-get update && apt-get install -y curl libunwind8 gettext

tools/dockerfile/test/csharp_jessie_x64/Dockerfile

@@ -80,6 +80,8 @@ RUN apt-get update && apt-get -y dist-upgrade && apt-get install -y \
     nuget \
     && apt-get clean
 
+RUN nuget update -self
+
 # Prepare ccache
 RUN ln -s /usr/bin/ccache /usr/local/bin/gcc
 RUN ln -s /usr/bin/ccache /usr/local/bin/g++

tools/dockerfile/test/multilang_jessie_x64/Dockerfile

@@ -80,6 +80,8 @@ RUN apt-get update && apt-get -y dist-upgrade && apt-get install -y \
     nuget \
     && apt-get clean
 
+RUN nuget update -self
+
 #=================
 # C++ dependencies
 RUN apt-get update && apt-get -y install libgflags-dev libgtest-dev libc++-dev clang && apt-get clean