ci: Clean up disk usage before saving container to disk (#34770)

Saving the Docker image to disk is failing on EE `pg` because of lack of
disk space. So we clean up all the unused Docker images just before
doing this.

.github/workflows/build-docker-image.yml

@@ -113,6 +113,10 @@ jobs:
       # Saving the docker image to tar file
       - name: Save Docker image to tar file
         run: |
+          docker image ls --all --no-trunc --format '{{.Repository}},{{.ID}}' \
+            | grep -v cicontainer \
+            | cut -d, -f2 \
+            | xargs docker rmi
           docker save cicontainer -o cicontainer.tar
           gzip cicontainer.tar