fix(ci): expose the dind docker host to job containers

gitea/runner 1.x no longer auto-mounts the docker daemon into job
containers (act_runner 0.2.x did), so 'docker build' in the build jobs
failed with 'cannot connect to /var/run/docker.sock'. container.docker_host
"" restores find-and-mount.

infrastructure/production/fleet/ci/gitea-runner.yaml

@@ -49,6 +49,12 @@ data:
       # We don't use the Actions cache (setup-node cache was removed); disabling
       # avoids the cache server the DinD job containers can't reach anyway.
       enabled: false
+    container:
+      # "" = find an available docker host automatically AND expose it to job
+      # containers (the dind daemon's socket gets bind-mounted into jobs, which
+      # is what `docker build` in the build jobs needs). gitea/runner 1.x no
+      # longer does this by default — act_runner 0.2.x did.
+      docker_host: ""
 ---
 apiVersion: apps/v1
 kind: Deployment