gha/build: Publish bin image for release branches

We moved to the major release branches with a `.x` suffix and forgot to
adjust this workflow to run on branches like `27.x`.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

Author: vvoland

.github/workflows/build.yml

@@ -22,6 +22,7 @@ on:
     branches:
       - 'master'
       - '[0-9]+.[0-9]+'
+      - '[0-9]+.x'
     tags:
       - 'v*'
   pull_request: