feat: add Dockerfile, Gitea Actions CI, and Artifact Hub metadata

Multi-stage Dockerfile produces a lightweight image with the compiled
plugin at /plugins/. Gitea Actions workflow triggers on tag push (v*),
builds the plugin, pushes to the Gitea container registry, and creates
a release with the tarball attached.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,9 @@
+FROM node:20-alpine AS build
+WORKDIR /app
+COPY package.json package-lock.json* ./
+RUN npm ci --ignore-scripts
+COPY src/ src/
+RUN npx @kinvolk/headlamp-plugin build
+
+FROM alpine:3.20
+COPY --from=build /app/dist/ /plugins/polaris-headlamp-plugin/