Add production deployment config (Caddy + docker compose)

.env.prod.example

@@ -0,0 +1,15 @@
+# Copy this file to .env on the server, next to docker-compose.prod.yml,
+# and fill in real values. NEVER commit the real .env file.
+#
+# Generate strong secrets with:
+#   openssl rand -hex 24   -> POSTGRES_PASSWORD
+#   openssl rand -hex 32   -> JWT_SECRET
+
+POSTGRES_DB=sig
+POSTGRES_USER=sig
+POSTGRES_PASSWORD=
+JWT_SECRET=
+
+# Public hostname Caddy will request a Let's Encrypt cert for.
+# No domain yet? Use <server-ip>.sslip.io - it resolves to the server's IP.
+DOMAIN=