chore: remove custom runner Dockerfiles and build workflow

No longer needed — runner labels now point to official language images
(golang:1.26-bookworm, node:22-bookworm, rust:1-bookworm) which already
have the language tools. Docker CLI installed via apt-get in workflow steps.

.gitea/workflows/build-runner-images.yml

@@ -1,65 +0,0 @@
-name: Build and push runner images
-
-on:
-  push:
-    paths:
-      - 'k8s/infra/forgejo-runner/Dockerfile.golang'
-      - 'k8s/infra/forgejo-runner/Dockerfile.rust'
-      - 'k8s/infra/forgejo-runner/Dockerfile.node'
-    branches:
-      - main
-  pull_request:
-    paths:
-      - 'k8s/infra/forgejo-runner/Dockerfile.golang'
-      - 'k8s/infra/forgejo-runner/Dockerfile.rust'
-      - 'k8s/infra/forgejo-runner/Dockerfile.node'
-      - '.gitea/workflows/build-runner-images.yml'
-
-jobs:
-  build-runners:
-    runs-on: golang
-    # Unified CI pattern: golang label → golang:1.26-bookworm (Debian, root, apt-get)
-    # Install Node.js + docker in steps. actions/checkout@v4 needs Node.js.
-    steps:
-      - name: Install dependencies
-        run: |
-          apt-get update
-          apt-get install -y --no-install-recommends nodejs docker.io
-          rm -rf /var/lib/apt/lists/*
-
-      - uses: actions/checkout@v4
-
-      - name: Get commit SHA
-        id: sha
-        run: |
-          SHORT_SHA=$(git rev-parse --short HEAD)
-          echo "short_sha=${SHORT_SHA}" >> $GITHUB_OUTPUT
-
-      - name: Build all runner images
-        run: |
-          set -e
-          for RUNNER in golang rust node; do
-            echo "📦 Building ${RUNNER}-runner..."
-            docker build -f "k8s/infra/forgejo-runner/Dockerfile.${RUNNER}" \
-              -t "${IMAGE_BASE}/forgejo-runner-${RUNNER}:${{ steps.sha.outputs.short_sha }}" \
-              -t "${IMAGE_BASE}/forgejo-runner-${RUNNER}:latest" \
-              .
-            echo "✅ Built ${RUNNER}-runner"
-          done
-
-      - name: Push images (main only)
-        if: github.event_name == 'push'
-        run: |
-          echo "${REGISTRY_TOKEN}" | docker login "${REGISTRY}" \
-            --username "${REGISTRY_USER}" --password-stdin
-          for RUNNER in golang rust node; do
-            docker push "${IMAGE_BASE}/forgejo-runner-${RUNNER}:${{ steps.sha.outputs.short_sha }}"
-            docker push "${IMAGE_BASE}/forgejo-runner-${RUNNER}:latest"
-            echo "✅ Pushed ${RUNNER}-runner"
-          done
-        env:
-          REGISTRY_USER: ${{ secrets.FORGEJO_REGISTRY_USER }}
-          REGISTRY_TOKEN: ${{ secrets.FORGEJO_REGISTRY_TOKEN }}
-    env:
-      REGISTRY: forgejo.riotpiao.com
-      IMAGE_BASE: forgejo.riotpiao.com/rock

k8s/infra/forgejo-runner/Dockerfile.golang

@@ -1,16 +0,0 @@
-FROM code.forgejo.org/forgejo/runner:6
-
-# Switch to root to install packages (Alpine)
-USER root
-
-# Alpine uses apk, not apt-get
-RUN apk update && apk add --no-cache \
-      nodejs \
-      npm \
-      docker
-
-# Verify installations
-RUN docker --version && node --version && git --version
-
-# Switch back to runner user
-USER 1000:1000

k8s/infra/forgejo-runner/Dockerfile.node

@@ -1,16 +0,0 @@
-FROM code.forgejo.org/forgejo/runner:6
-
-# Switch to root to install packages (Alpine)
-USER root
-
-# Alpine uses apk, not apt-get
-RUN apk update && apk add --no-cache \
-      docker \
-      nodejs \
-      npm
-
-# Verify installations
-RUN node --version && docker --version && git --version
-
-# Switch back to runner user
-USER 1000:1000

k8s/infra/forgejo-runner/Dockerfile.rust

@@ -1,20 +0,0 @@
-FROM code.forgejo.org/forgejo/runner:6
-
-# Switch to root to install packages (Alpine)
-USER root
-
-# Alpine uses apk, not apt-get
-RUN apk update && apk add --no-cache \
-      nodejs \
-      npm \
-      curl \
-      docker
-
-# Install Rust (as root, skip verification for now)
-RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable || true
-
-# Verify core installations
-RUN docker --version && node --version && git --version
-
-# Switch back to runner user
-USER 1000:1000