[FlINK-36332] Default to okhttp if client not specified

Author: SamBarker

.github/workflows/ci.yml

@@ -197,7 +197,8 @@ jobs:
           export SHELL=/bin/bash
           export DOCKER_BUILDKIT=1
           eval $(minikube -p minikube docker-env)
-          docker build --progress=plain --no-cache -f ./Dockerfile -t flink-kubernetes-operator:ci-${{ matrix.http-client }}-latest --progress plain --build-arg HTTP_CLIENT=${{ matrix.http-client }} .
+          HTTP_CLIENT=${{ matrix.http-client }}
+          docker build --progress=plain --no-cache -f ./Dockerfile -t flink-kubernetes-operator:ci-${{ matrix.http-client }}-latest --progress plain --build-arg HTTP_CLIENT="${HTTP_CLIENT:-okhttp}" .
           docker images
       - name: Start the operator
         run: |