Enhancement (ci): Use separate workflow for PR docker builds

This is so the `docker` workflow status is not polluted with PR builds.

.github/workflows/docker.yml

@@ -3,10 +3,7 @@ name: docker
 on:
   push:
     tags:
-    - '**'
-  pull_request:
-    branches:
-    - master
+    - 'v2.*'
 
 # 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:
@@ -31,12 +28,9 @@ 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
         # The rest of the org.opencontainers.image.xxx labels are dynamically generated