Fix invalid Docker tag on PR events by using static sha- prefix

Co-authored-by: ThisIs-Developer <109382325+ThisIs-Developer@users.noreply.github.com>

Author: Copilot

.github/workflows/docker-publish.yml

@@ -39,7 +39,7 @@ jobs:
         tags: |
           type=ref,event=branch
           type=ref,event=pr
-          type=sha,prefix={{branch}}-
+          type=sha,prefix=sha-
           type=raw,value=latest,enable={{is_default_branch}}
 
     - name: Build and push Docker image