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

.dockerignore

@@ -0,0 +1,11 @@
+.git
+.github
+node_modules
+dist
+.astro
+.vscode
+.idea
+*.log
+.DS_Store
+README.md
+LICENSE