initial: flutter-node runner image

Custom CI runner image combining cirruslabs/flutter:stable with Node 20
from the official node image, so Gitea Actions runners can execute
JS-based actions (actions/checkout, actions/upload-artifact, etc.) on
Flutter projects.

Registry host is configurable via the vars.REGISTRY_HOST Gitea variable;
weekly cron picks up upstream patch updates without source changes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Dockerfile

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+# Generic CI runner image: cirruslabs/flutter base + Node from the
+# official node image, so Gitea Actions / GitHub Actions runners can
+# execute JS-based actions (actions/checkout, actions/upload-artifact,
+# actions/setup-*) on Flutter projects. Vanilla cirruslabs/flutter
+# ships no Node, which is why pipelines using those actions fail with
+# `exec: "node": executable file not found in $PATH`.
+#
+# Bases are pinned via build-args. Bump deliberately by editing this
+# file; the weekly cron in .gitea/workflows/build.yaml picks up
+# Cirrus's :stable refresh (and Node 20.x patch updates) automatically
+# without a source change.
+
+ARG FLUTTER_IMAGE=ghcr.io/cirruslabs/flutter:stable
+ARG NODE_IMAGE=node:20-bookworm-slim
+ARG IMAGE_SOURCE=
+
+FROM ${NODE_IMAGE} AS node
+
+FROM ${FLUTTER_IMAGE}
+ARG IMAGE_SOURCE
+
+# Links the published OCI package to its source repo so Gitea inherits
+# visibility (public repo → publicly pullable image, no auth needed).
+# IMAGE_SOURCE is supplied at build time from vars.REGISTRY_HOST.
+LABEL org.opencontainers.image.source="${IMAGE_SOURCE}"
+LABEL org.opencontainers.image.description="Flutter (cirruslabs/flutter:stable) + Node 20, for CI runners that execute JS-based actions."
+LABEL org.opencontainers.image.licenses="MIT"
+
+# Bring node + npm + npx in from the official node image. Symlinks
+# mirror what `npm install -g` does on a normal install.
+COPY --from=node /usr/local/bin/node          /usr/local/bin/node
+COPY --from=node /usr/local/lib/node_modules  /usr/local/lib/node_modules
+RUN ln -sf /usr/local/lib/node_modules/npm/bin/npm-cli.js /usr/local/bin/npm \
+ && ln -sf /usr/local/lib/node_modules/npm/bin/npx-cli.js /usr/local/bin/npx
+
+# Sanity gate: a missing binary fails the build rather than silently
+# shipping a broken image.
+RUN node --version && npm --version && npx --version && flutter --version