build: add Dockerfile + nginx config for self-hosted deploy (#5)

Multi-stage build (node:22-alpine -> nginx:1.27-alpine).
Verified locally on townhouse-remote: 4 pages return 200,
security headers present (X-Frame, X-Content-Type, Referrer,
Permissions), cache strategy via Content-Type map (HTML no-store,
CSS/JS/fonts 1y immutable, images 30d).

Deploy target: Coolify on townhouse-remote, container port 80
mapped to host 9200 (reserved 9200-9299 for imagesbyolofsson.se
stack).

Includes:
- Dockerfile      Multi-stage build with healthcheck
- nginx.conf      Content-type-driven cache + security headers
- .dockerignore   Excludes node_modules, .git, dist, .astro

Dockerfile

@@ -0,0 +1,20 @@
+# syntax=docker/dockerfile:1.7
+
+FROM node:22-alpine AS builder
+WORKDIR /app
+
+COPY package.json package-lock.json* ./
+RUN --mount=type=cache,target=/root/.npm npm ci
+
+COPY . .
+RUN npm run build
+
+FROM nginx:1.27-alpine AS runtime
+COPY --from=builder /app/dist /usr/share/nginx/html
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+
+RUN adduser -D -H -u 101 -s /sbin/nologin nginxuser 2>/dev/null || true
+EXPOSE 80
+
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
+  CMD wget -q --spider http://127.0.0.1/ || exit 1