feat: add Docker CI image with pre-baked toolchain + deps

Dockerfile.ci pre-installs bun, node, claude CLI, gh CLI, and
node_modules so eval runners skip all setup. Image rebuilds weekly
and on lockfile/Dockerfile changes via ci-image.yml.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/docker/Dockerfile.ci

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+# gstack CI eval runner — pre-baked toolchain + deps
+# Rebuild weekly via ci-image.yml, on Dockerfile changes, or on lockfile changes
+FROM ubuntu:24.04
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+# System deps
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    git curl unzip ca-certificates jq bc gpg \
+    && rm -rf /var/lib/apt/lists/*
+
+# GitHub CLI
+RUN curl -fsSL https://cli.github.com/packages/githubcli-archive-keyring.gpg \
+    | gpg --dearmor -o /usr/share/keyrings/githubcli-archive-keyring.gpg \
+    && echo "deb [arch=$(dpkg --print-architecture) signed-by=/usr/share/keyrings/githubcli-archive-keyring.gpg] https://cli.github.com/packages stable main" \
+    | tee /etc/apt/sources.list.d/github-cli.list > /dev/null \
+    && apt-get update && apt-get install -y --no-install-recommends gh \
+    && rm -rf /var/lib/apt/lists/*
+
+# Node.js 22 LTS (needed for claude CLI)
+RUN curl -fsSL https://deb.nodesource.com/setup_22.x | bash - \
+    && apt-get install -y --no-install-recommends nodejs \
+    && rm -rf /var/lib/apt/lists/*
+
+# Bun
+RUN curl -fsSL https://bun.sh/install | bash
+ENV BUN_INSTALL="/root/.bun"
+ENV PATH="$BUN_INSTALL/bin:$PATH"
+
+# Claude CLI
+RUN npm i -g @anthropic-ai/claude-code
+
+# Pre-install dependencies (cached layer — only rebuilds when lockfile changes)
+COPY bun.lockb package.json /workspace/
+WORKDIR /workspace
+RUN bun install --frozen-lockfile && rm -rf /tmp/*
+
+# Verify everything works
+RUN bun --version && node --version && claude --version && jq --version && gh --version
+
+# At runtime: checkout overwrites /workspace, but node_modules persists
+# if we move it out of the way and symlink back
+RUN mv /workspace/node_modules /opt/node_modules_cache

.github/workflows/ci-image.yml

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+name: Build CI Image
+on:
+  # Rebuild weekly (Monday 6am UTC) to pick up CLI updates
+  schedule:
+    - cron: '0 6 * * 1'
+  # Rebuild on Dockerfile or lockfile changes
+  push:
+    branches: [main]
+    paths:
+      - '.github/docker/Dockerfile.ci'
+      - 'bun.lockb'
+      - 'package.json'
+  # Manual trigger
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: ubicloud-standard-2
+    permissions:
+      contents: read
+      packages: write
+    steps:
+      - uses: actions/checkout@v4
+
+      # Copy lockfile + package.json into Docker build context
+      - run: cp bun.lockb package.json .github/docker/
+
+      - uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - uses: docker/build-push-action@v6
+        with:
+          context: .github/docker
+          file: .github/docker/Dockerfile.ci
+          push: true
+          tags: |
+            ghcr.io/${{ github.repository }}/ci:latest
+            ghcr.io/${{ github.repository }}/ci:${{ github.sha }}