ci: force bash as default shell in container jobs

When jobs run with `container:` set, Gitea Actions defaults to
`sh -e` (dash on Ubuntu), so `set -euo pipefail` fails with
"Illegal option -o pipefail". Pinning bash workflow-wide
matches what the runner used pre-container and keeps existing
scripts portable.

.gitea/workflows/release.yml

@@ -37,6 +37,11 @@ env:
   REGISTRY: gitea.dcglab.co.uk
   IMAGE_NAME: ${{ gitea.repository }}
 
+# Force bash as the default shell — see ci.yml header.
+defaults:
+  run:
+    shell: bash
+
 jobs:
   image:
     name: Build + push image