feat: add Docker support for server deployment

Add Docker deployment option for the server with multi-stage build.

Changes:
- Dockerfile: multi-stage build (golang:1.26-alpine → alpine:3.21, ~23MB)
- docker-compose.yml: server service on port 5300/udp with iptables redirect
- .env.example: documented environment variables template
- .dockerignore: optimized build context
- README.md & README-FA.md: Docker deployment guide, port 53 safety checks,
  and troubleshooting instructions (both EN and FA)
- configs: added more Telegram channels and X accounts

The container listens on port 5300 to avoid conflict with systemd-resolved.
External DNS traffic (port 53) is redirected via iptables PREROUTING.
No changes to existing Go source code.

.env.example

@@ -0,0 +1,40 @@
+# ============================================================
+# thefeed-server — Docker Environment Variables
+# ============================================================
+# Copy this file to .env and fill in your values:
+#   cp .env.example .env
+# ============================================================
+
+# ----- Required -----
+
+# Your DNS domain (e.g., t.example.com)
+THEFEED_DOMAIN=t.example.com
+
+# Encryption passphrase (shared between server and client)
+THEFEED_KEY=your-secret-passphrase
+
+# ----- Telegram Mode -----
+# Default: --no-telegram (public channels only, no login needed)
+# To use Telegram API, uncomment these AND remove --no-telegram from compose:
+
+#TELEGRAM_API_ID=12345
+#TELEGRAM_API_HASH=your-api-hash
+#TELEGRAM_PHONE=+1234567890
+#TELEGRAM_PASSWORD=your-2fa-password
+
+# ----- Optional Settings -----
+
+# Max messages per channel (default: 15)
+#THEFEED_MSG_LIMIT=15
+
+# Allow remote channel management via DNS (default: disabled)
+#THEFEED_ALLOW_MANAGE=0
+
+# Nitter RSS instances for X/Twitter (comma-separated)
+#THEFEED_X_RSS_INSTANCES=http://nitter.net,https://nitter.net
+
+# Max random padding bytes in DNS responses (anti-DPI, default: 32)
+#THEFEED_PADDING=32
+
+# Log every decoded DNS query (default: disabled)
+#THEFEED_DEBUG=0