fix: use bash shell for string slicing in image tag computation

The pinned container image uses sh which doesn't support bash
string slicing syntax (${var:0:7}). Explicitly set shell: bash
to ensure compatibility.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitea/workflows/build-push.yaml

@@ -58,6 +58,7 @@ jobs:
 
       - name: Compute image tag
         id: tag
+        shell: bash
         run: |
           SHORT_SHA="${{ gitea.sha }}"
           SHORT_SHA="${SHORT_SHA:0:7}"