Add Dockerfile and docker-compose for production deployment

Multi-stage build: hugomods/hugo:exts → nginx:alpine.
Runs on port 8080.

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

Dockerfile

@@ -0,0 +1,9 @@
+FROM hugomods/hugo:exts AS builder
+WORKDIR /src
+COPY . .
+RUN git submodule update --init --recursive && \
+    hugo --gc --minify
+
+FROM nginx:alpine
+COPY --from=builder /src/public /usr/share/nginx/html
+EXPOSE 80