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*'
 
@@ -20,7 +20,6 @@ jobs:
         with:
           images: la5nta/pat
           tags: |
-            type=ref,event=branch
             type=semver,pattern={{version}}
             type=semver,pattern={{major}}.{{minor}}
       - name: Set up QEMU