fix: Forgejo CI runners — proper images, shared docker socket, auto-registration

All CI workflows across all repos were broken. Three root causes:

1. Runner labels pointed to bare Alpine image (forgejo/runner:6) which has
   no Node.js, no docker, no Go/Rust, no root, no apt-get. Every workflow
   step failed immediately.

   Fix: Change labels to official Debian language images:
     golang → docker://golang:1.26-bookworm
     node   → docker://node:22-bookworm
     rust   → docker://rust:1-bookworm

2. Docker socket not shared between dind sidecar and runner container.
   dind creates /run/docker.sock in its own filesystem. Runner container
   has a separate filesystem and can't see it. Workflow containers that
   run 'docker build' get 'Cannot connect to Docker daemon'.

   Fix: Share /run between dind and runner via emptyDir volume.
   Add docker_host: automount to runner config so the socket is
   mounted into workflow containers automatically.

   Note: /var/run is a symlink to /run in Alpine — must mount at /run.

3. Init container skipped re-registration if .runner file existed on PVC.
   Changing labels in values.yaml had no effect until PVC was manually
   deleted — not GitOps-friendly.

   Fix: Always rm .runner and re-register on pod start. Labels stay
   in sync with values.yaml automatically.

Also removes custom Dockerfiles and build-runner-images workflow — no longer
needed since official images provide the language tools directly.

k8s/infra/forgejo-runner/values-rust.yaml

@@ -2,13 +2,14 @@
 # runner instance. Only runner.name and runner.labels differ -- everything
 # else (image, dind, persistence, tolerations, nodeSelector) is shared.
 #
-# Bootstrap with runner image, CI builds custom with Node.js+Rust
+# Label image: rust:1-bookworm — Debian, root, apt-get, Rust, cargo, git.
+# Install Node.js/docker in workflow steps as needed.
 runner:
   image:
     repository: code.forgejo.org/forgejo/runner
     tag: "6"
   name: rust-runner
-  labels: "rust:docker://code.forgejo.org/forgejo/runner:6"
+  labels: "rust:docker://rust:1-bookworm"