CI: target the shell runner label instead of docker

The only registered act_runner advertises the `shell` label, not
`docker`, so `runs-on: docker` left the job queued forever. The host
has the docker CLI (the resume repo builds via `docker run` on this
same runner), so keep running the build/test inside a golang container.

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

.forgejo/workflows/build.yml

@@ -2,7 +2,7 @@ name: Build and Test
 on: [push]
 jobs:
   build-and-test:
-    runs-on: docker
+    runs-on: shell
     steps:
       - name: Checkout
         run: |