ci: Only build custom images that need to be rebuilt

Signed-off-by: Rémy Coutable <remy@rymai.me>

.gitlab/ci/customers.images.yml

@@ -1,7 +1,16 @@
+.customers:
+  extends:
+    - .build_dynamic_image
+  rules:
+    - !reference [.build_dynamic_image, rules]
+    - changes:
+        - "scripts/lib/custom-docker-build"
+        - ".gitlab/ci/customers.images.yml"
+
 # Used by customers-gitlab-com rspec job: https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/staging/.gitlab-ci.yml
 customers:
   extends:
-    - .build_and_push
+    - .customers
   variables:
     LFS: '2.9'
     CHROME: '107'
@@ -17,7 +26,7 @@ customers:
 # Used by customers-gitlab-com qa e2e test: https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/staging/.gitlab/ci/e2e.gitlab-ci.yml
 customers-qa-assets:
   extends:
-    - .build_and_push
+    - .customers
   variables:
     CHROME: '107'
     NODE: '16.14'