feat: Dockerfile and docker-compose

Dockerfile: node:20-alpine, exposes 3456, STORE_PATH=/app/data/store.json.
docker-compose.yml: proxy service with ./data volume mount + inline comments
showing how to wire GOCARDLESS_BASE_URL into an existing actual-server compose.

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

Dockerfile

@@ -0,0 +1,20 @@
+FROM node:20-alpine
+
+WORKDIR /app
+
+# Install dependencies first (layer cache)
+COPY package.json ./
+RUN npm install --omit=dev
+
+# Copy source
+COPY src/ ./src/
+
+# Data directory (override with a volume)
+RUN mkdir -p /app/data
+
+EXPOSE 3456
+
+ENV NODE_ENV=production
+ENV STORE_PATH=/app/data/store.json
+
+CMD ["node", "src/server.js"]