Trac #33339: Use latest dev docker build for documentation workflow

Follow up from #31415. Forgot to change from `9.5` to `dev` docker tag
before merge. Mea culpa.

URL: https://trac.sagemath.org/33339
Reported by: gh-tobiasdiez
Ticket author(s): Tobias Diez
Reviewer(s): Matthias Koeppe

.github/workflows/doc-build.yml

@@ -13,7 +13,7 @@ concurrency:
 jobs:
   build-docs:
     runs-on: ubuntu-latest
-    container: ghcr.io/sagemath/sage/sage-docker-ubuntu-focal-standard-with-targets:9.5
+    container: ghcr.io/sagemath/sage/sage-docker-ubuntu-focal-standard-with-targets:dev
     if: github.repository == 'sagemath/sagetrac-mirror'
     steps:
       - name: Checkout