Fix invalid Docker tag format in workflow

Remove problematic prefix={{branch}}- from sha tag type that was
generating invalid tags like "-e007166" when branch context is empty.
Using simple "type=sha" instead which generates valid tags.

Co-authored-by: lpcox <15877973+lpcox@users.noreply.github.com>

Author: Copilot

.github/workflows/serena-container.yml

@@ -48,7 +48,7 @@ jobs:
           tags: |
             type=raw,value=latest,enable={{is_default_branch}}
             type=ref,event=pr
-            type=sha,prefix={{branch}}-
+            type=sha
             type=raw,value=${{ github.event.inputs.version }},enable=${{ github.event.inputs.version != '' }}
 
       - name: Build and push (multi-arch)