Add Docker self-hosting and CI/CD pipeline

- Dockerfile: Caddy + PHP-FPM + app in single Alpine container
- Caddyfile: auto-HTTPS, security headers, short URL rewrite
- docker-compose.yml: app + Watchtower for auto-updates
- install.sh: one-liner for fresh VPS setup
- GitHub Actions: build & push to Docker Hub + GHCR on tag

Self-host with:
  curl -sL https://xmrpay.link/install.sh | sh -s your-domain.com

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-entrypoint.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+set -e
+
+# Start PHP-FPM in background
+php-fpm &
+
+# Run Caddy in foreground
+exec caddy run --config /etc/caddy/Caddyfile --adapter caddyfile