fix: share docker socket between dind and runner via emptyDir

ROOT CAUSE: Docker socket (/var/run/docker.sock) only existed inside the
dind container — the runner container couldn't see it. The runner connected
to dind via TCP (tcp://localhost:2376) with TLS. But workflow containers
created by the runner had NO way to access the docker daemon:
- unix socket not mounted (runner can't see it)
- DOCKER_HOST env var not passed (runner.envs not configured)

FIX: Share /var/run between dind and runner via emptyDir volume.
When dind starts, it creates /var/run/docker.sock in the shared volume.
Runner can now see the socket. docker_host: automount in config tells
the runner to mount the socket into job containers automatically.

Architecture after fix:
  dind container → creates /var/run/docker.sock → shared emptyDir
  runner container → sees /var/run/docker.sock → uses automount
  workflow container → gets /var/run/docker.sock mounted by runner

Also removed runner.envs (TCP+TLS approach) — unix socket is simpler
and works with automount.

k8s/infra/forgejo-runner/templates/configmap.yaml

@@ -32,12 +32,8 @@ metadata:
   namespace: {{ .Release.Namespace }}
 data:
   config.yaml: |
-    runner:
-      envs:
-        DOCKER_HOST: tcp://localhost:2376
-        DOCKER_TLS_VERIFY: "1"
-        DOCKER_CERT_PATH: /docker-certs/client
     container:
       valid_volumes:
         - /docker-certs/client
       network: host
+      docker_host: automount