feat: use official singularity image

This would remove the need to maintain a special dind image.

.gitlab-ci.yml

 image: eicweb.phy.anl.gov:4567/containers/image_recipes/ubuntu_dind:latest
 
 variables:
+  ## External images
+  DOCKER_IMAGE: docker.io/docker:24.0.5
+  SINGULARITY_IMAGE: quay.io/singularity/singularity:v3.11.4
+
   ## Application versions used for the main release 
   ## note: nightly builds will always use the master/main branch
   JUGGLER_VERSION: "v10.0.1"
@@ -407,11 +411,14 @@ jug_xl:nightly:
                    containers/jug
 
 .singularity:
+  image:
+    name: ${SINGULARITY_IMAGE}
+    entrypoint: ["/bin/sh"]
   stage: deploy
   interruptible: true
   rules:
     - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
-      when: never
+      when: always
   artifacts:
       expire_in: 1 days
       paths: