diff --git a/tools/dockerfile/distribtest/csharp_dotnet5_x64/Dockerfile b/tools/dockerfile/distribtest/csharp_dotnet5_x64/Dockerfile
index 60ce55b5b9d..3f95ef1134c 100644
--- a/tools/dockerfile/distribtest/csharp_dotnet5_x64/Dockerfile
+++ b/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