Run build on PR but don't push

.github/workflows/docker.yml

@@ -3,7 +3,10 @@ name: docker
 on:
   push:
     tags:
-    - 'v2.*'
+    - '**'
+  pull_request:
+    branches:
+    - master
 
 # This is needed to push to GitHub Container Registry. See https://docs.github.com/en/packages/working-with-a-github-packages-registry/working-with-the-container-registry
 permissions:
@@ -28,9 +31,11 @@ jobs:
         images: |
           djmaze/snappymail
           ghcr.io/${{ github.repository }}
+        # type=ref,event=pr generates tag(s) on PRs only. E.g. 'pr-123', 'pr-123-abc0123'
         # type=ref,event=branch generates tag(s) on branch only. E.g. 'master', 'master-abc0123'
         # type=ref,event=tag generates tag(s) on tags only. E.g. 'v0.0.0', 'v0.0.0-abc0123', and 'latest'
         tags: |
+          type=ref,event=pr
           type=ref,suffix=-{{sha}},event=pr
           type=ref,event=branch
           type=ref,event=tag
@@ -101,7 +106,7 @@ jobs:
         # TODO: Add more arches?
         # platforms: linux/386,linux/amd64,linux/arm/v6,linux/arm/v7,linux/arm64,linux/s390x
         platforms: linux/386,linux/amd64
-        push: true
+        push: startsWith(github.ref, 'refs/tags/')  # Push only on tags
         tags: ${{ steps.meta.outputs.tags }}
         labels: ${{ steps.meta.outputs.labels }}
         cache-from: type=local,src=/tmp/.buildx-cache