only push docker descriptions on master branch pipelines (#13664)

Currently those jobs also get included in multi-project pipelines such
as https://gitlab.parity.io/parity/mirrors/scripts/-/pipelines/255546,
even when the description hasn't actually changed, because `changes:`
conditions always evaluate to true on non-branch pipelines.

substrate/scripts/ci/gitlab/pipeline/publish.yml

@@ -59,7 +59,7 @@
     DOCKER_PASSWORD: $Docker_Hub_Pass_Parity
     README_FILEPATH: $CI_PROJECT_DIR/scripts/ci/docker/$PRODUCT.Dockerfile.README.md
   rules:
-    - if: $CI_COMMIT_REF_NAME == "master"
+    - if: $CI_COMMIT_REF_NAME == $CI_DEFAULT_BRANCH && $CI_PIPELINE_SOURCE == "push"
       changes:
         - scripts/ci/docker/$PRODUCT.Dockerfile.README.md
   before_script: