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/ci.yml

@@ -54,6 +54,13 @@ on:
   pull_request:
     branches: [main]
 
+# Force bash as the default shell. With `container:` set on every
+# job, Gitea Actions otherwise picks `sh -e` and our `set -euo
+# pipefail` fails on dash with "Illegal option -o pipefail".
+defaults:
+  run:
+    shell: bash
+
 jobs:
   test:
     # Sharded by package group. server/http and store are the two