Update xDS client/server image per-branch tag after build (#26955)

* Update xDS client/server image per-branch tag after build

* Address reviewers' comments

* Replace the entire string instead of only prefix

* Update regex

* Update tools/internal_ci/linux/grpc_xds_k8s_install_test_driver.sh

Co-authored-by: Sergii Tkachenko <hi@sergii.org>

Co-authored-by: Sergii Tkachenko <hi@sergii.org>

tools/internal_ci/linux/grpc_xds_k8s.sh

@@ -45,6 +45,11 @@ build_test_app_docker_images() {
   gcloud -q auth configure-docker
   docker push "${CLIENT_IMAGE_NAME}:${GIT_COMMIT}"
   docker push "${SERVER_IMAGE_NAME}:${GIT_COMMIT}"
+  if [[ -n $KOKORO_JOB_NAME ]]; then
+    branch_name=$(echo "$KOKORO_JOB_NAME" | sed -E 's|^grpc/core/([^/]+)/.*|\1|')
+    tag_and_push_docker_image "${CLIENT_IMAGE_NAME}" "${GIT_COMMIT}" "${branch_name}"
+    tag_and_push_docker_image "${SERVER_IMAGE_NAME}" "${GIT_COMMIT}" "${branch_name}"
+  fi
 }
 
 #######################################

tools/internal_ci/linux/grpc_xds_k8s_install_test_driver.sh

@@ -361,3 +361,20 @@ local_setup_test_driver() {
   mkdir -p "${TEST_XML_OUTPUT_DIR}"
 }
 
+#######################################
+# Tag and push the given Docker image
+# Arguments:
+#   The Docker image name
+#   The Docker image original tag name
+#   The Docker image new tag name
+# Outputs:
+#   Writes the output to stdout, stderr, files
+#######################################
+tag_and_push_docker_image() {
+  local image_name="$1"
+  local from_tag="$2"
+  local to_tag="$3"
+
+  docker tag "${image_name}:${from_tag}" "${image_name}:${to_tag}"
+  docker push "${image_name}:${to_tag}"
+}

tools/internal_ci/linux/grpc_xds_k8s_python.sh

@@ -59,6 +59,12 @@ build_test_app_docker_images() {
 
   docker push "${CLIENT_IMAGE_NAME}:${GIT_COMMIT}"
   docker push "${SERVER_IMAGE_NAME}:${GIT_COMMIT}"
+
+  if [[ -n $KOKORO_JOB_NAME ]]; then
+    branch_name=$(echo "$KOKORO_JOB_NAME" | sed -E 's|^grpc/core/([^/]+)/.*|\1|')
+    tag_and_push_docker_image "${CLIENT_IMAGE_NAME}" "${GIT_COMMIT}" "${branch_name}"
+    tag_and_push_docker_image "${SERVER_IMAGE_NAME}" "${GIT_COMMIT}" "${branch_name}"
+  fi
 }
 
 #######################################