update dotnet sdk5 docker image to avoid nuget download error

tools/dockerfile/distribtest/csharp_dotnet5_x64/Dockerfile

@@ -12,8 +12,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-# "5.0" tag uses debian buster
-FROM mcr.microsoft.com/dotnet/sdk:5.0
+# "5.0.x" tag uses debian buster
+# we use the full version number to make docker image version updates explicit
+FROM mcr.microsoft.com/dotnet/sdk:5.0.103
 
 RUN apt-get update && apt-get install -y unzip && apt-get clean