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

docker-compose.yml

@@ -0,0 +1,27 @@
+services:
+  xmrpay:
+    image: ${XMRPAY_IMAGE:-schmidt1024/xmrpay:latest}
+    container_name: xmrpay
+    restart: unless-stopped
+    ports:
+      - "80:80"
+      - "443:443"
+    environment:
+      - DOMAIN=${DOMAIN:-localhost}
+    volumes:
+      - xmrpay-data:/srv/data
+      - caddy-data:/data/caddy
+
+  watchtower:
+    image: containrrr/watchtower
+    container_name: watchtower
+    restart: unless-stopped
+    environment:
+      - WATCHTOWER_CLEANUP=true
+      - WATCHTOWER_POLL_INTERVAL=21600
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock
+
+volumes:
+  xmrpay-data:
+  caddy-data: