ci: Push docker images only when new tag is pushed

... but run the pipeline on release/* branches, so we can verify that
they build before merging to master and tagging.

.github/workflows/docker.yaml

@@ -3,8 +3,8 @@ name: docker-push
 on:
   push:
     branches:
-      - 'docker'
-      - 'master'
+      - 'ci-test/*'
+      - 'release/*'
     tags:
       - 'v*'
 
@@ -37,6 +37,6 @@ jobs:
         with:
           context: .
           platforms: linux/amd64,linux/386,linux/arm64/v8,linux/arm/v7,linux/arm/v6
-          push: true
+          push: ${{ github.event_name == 'push' && startsWith(github.ref, 'refs/tags/v') }}
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}