ci(docs): run on the docker runner label, not ubuntu-latest

The docs workflow sat in "Waiting" indefinitely — "no online runner found
matching this label: ubuntu-latest". Our Forgejo runner advertises the
`docker` label (and is Docker-capable, so the container: python:3.12 step
still works); `ubuntu-latest` is a GitHub-ism it never registered. Point
runs-on at the label the runner actually has.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.forgejo/workflows/docs.yml

@@ -18,7 +18,11 @@ permissions:
 
 jobs:
   build-and-deploy:
-    runs-on: ubuntu-latest
+    # Match the label our Forgejo runner actually advertises. The runner is
+    # Docker-capable (it provides the `container:` below); `ubuntu-latest` is a
+    # GitHub-ism this runner does not register, so a job requesting it sits in
+    # "Waiting" forever with "no online runner found matching this label".
+    runs-on: docker
     # Run in a Python image: the generator is pure-Python stdlib, and this image
     # already has git for the clone + publish. No node actions are used, so the
     # job never depends on the runner's base image having python installed.