Remove unnecessary explicit false push

.gitlab/ci/custom.images.yml

@@ -8,7 +8,7 @@
       docker buildx build \
         --cache-to=type=inline \
         --cache-from="$CI_REGISTRY_IMAGE:$1" \
-        --push="${PUSH_CUSTOM_IMAGE}" \
+        --push="${PUSH_CUSTOM_IMAGE:-false}" \
         --label "ci_pipeline_url=$CI_PIPELINE_URL" \
         --label "ci_job_url=$CI_JOB_URL" \
         -t "$CI_REGISTRY_IMAGE:$1" \

.gitlab/ci/definitions.yml

@@ -5,8 +5,6 @@
     - ./scripts/lib/custom-docker-build
   rules:
     - if: $CI_PIPELINE_SOURCE == "merge_request_event"
-      variables:
-        PUSH_CUSTOM_IMAGE: "false"
     - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
       variables:
         PUSH_CUSTOM_IMAGE: "true"