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

nginx.conf

@@ -0,0 +1,46 @@
+map $sent_http_content_type $cache_control_strategy {
+    default                              "public, max-age=2592000";
+    ~^text/html                          "no-store, must-revalidate";
+    ~^text/css                           "public, max-age=31536000, immutable";
+    ~^application/javascript             "public, max-age=31536000, immutable";
+    ~^application/json                   "no-store, must-revalidate";
+    ~^image/                             "public, max-age=2592000";
+    ~^font/                              "public, max-age=31536000, immutable";
+}
+
+server {
+    listen 80 default_server;
+    server_name _;
+    root /usr/share/nginx/html;
+    index index.html;
+
+    add_header X-Content-Type-Options "nosniff" always;
+    add_header X-Frame-Options "SAMEORIGIN" always;
+    add_header Referrer-Policy "strict-origin-when-cross-origin" always;
+    add_header Permissions-Policy "geolocation=(), microphone=(), camera=()" always;
+    add_header Cache-Control $cache_control_strategy always;
+
+    location / {
+        try_files $uri $uri/ $uri.html $uri/index.html =404;
+    }
+
+    location /_astro/ {
+        access_log off;
+        expires 1y;
+    }
+
+    location ~* \.(webp|avif|jpg|jpeg|png|gif|svg|ico|woff2?|ttf|otf|eot)$ {
+        access_log off;
+        expires 30d;
+    }
+
+    gzip on;
+    gzip_vary on;
+    gzip_min_length 1024;
+    gzip_types text/plain text/css text/xml application/json application/javascript application/xml+rss image/svg+xml;
+
+    error_page 404 /404.html;
+    location = /404.html {
+        internal;
+    }
+}