[ci] Fix rules for docker build (#2069)

Author: alvicsam

.gitlab-ci.yml

@@ -56,7 +56,11 @@ default:
     - if: $CI_COMMIT_REF_NAME =~ /^[0-9]+$/                         # PRs
     - if: $CI_COMMIT_REF_NAME =~ /^v[0-9]+\.[0-9]+.*$/              # i.e. v1.0, v2.1rc1
 
-.publish-refs:                       &publish-refs
+.test-only-refs:                   &test-only-refs
+  rules:
+    - if: $CI_COMMIT_REF_NAME =~ /^[0-9]+$/                         # PRs
+
+.publish-refs:                     &publish-refs
   rules:
     # won't run on the CI image update pipeline
     - if: $CI_PIPELINE_SOURCE == "pipeline"
@@ -261,7 +265,7 @@ build-nightly:
 .build-image:                      &build-image
   <<:                              *kubernetes-build
   image:                           $BUILDAH_IMAGE
-  <<:                              *test-refs
+  <<:                              *test-only-refs
   variables:                       &build-image-variables
     GIT_STRATEGY:                  none
     DOCKERFILE:                    ci.Dockerfile