Add Docker build and Gitea Actions CI for deployment

Multi-stage Alpine Dockerfile producing a standalone Next.js image
with the better-sqlite3 native addon included. Gitea workflow builds
on push to main and on v* tags, pushing to the Gitea registry using
the REGISTRY_URL variable and REGISTRY_TOKEN secret. Compose file
mounts ./data so alerts.db and settings.json persist across restarts.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,39 @@
+# syntax=docker/dockerfile:1.7
+ARG NODE_VERSION=22-alpine
+
+# ── deps: install node_modules (with native build tools for better-sqlite3) ──
+FROM node:${NODE_VERSION} AS deps
+RUN apk add --no-cache libc6-compat python3 make g++
+WORKDIR /app
+COPY package.json package-lock.json ./
+RUN npm ci
+
+# ── builder: produce the standalone Next.js bundle ──
+FROM node:${NODE_VERSION} AS builder
+WORKDIR /app
+COPY --from=deps /app/node_modules ./node_modules
+COPY . .
+ENV NEXT_TELEMETRY_DISABLED=1
+RUN npm run build
+
+# ── runner: minimal runtime image ──
+FROM node:${NODE_VERSION} AS runner
+WORKDIR /app
+ENV NODE_ENV=production
+ENV NEXT_TELEMETRY_DISABLED=1
+ENV PORT=3000
+ENV HOSTNAME=0.0.0.0
+
+RUN addgroup --system --gid 1001 nodejs \
+ && adduser --system --uid 1001 nextjs
+
+COPY --from=builder /app/public ./public
+COPY --from=builder --chown=nextjs:nodejs /app/.next/standalone ./
+COPY --from=builder --chown=nextjs:nodejs /app/.next/static ./.next/static
+
+RUN mkdir -p /app/data && chown nextjs:nodejs /app/data
+
+USER nextjs
+EXPOSE 3000
+VOLUME ["/app/data"]
+CMD ["node", "server.js"]