Added L2TP/IPSec VPN Server Docker Image

Signed-off-by: Teddysun <i@teddysun.com>

docker/l2tp/README.md

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+## L2TP/IPsec VPN Server Docker Image by Teddysun
+
+Docker image to run a L2TP/IPsec VPN Server, with both `L2TP/IPsec PSK` and `IPSec Xauth PSK`.
+Based on Debian 9 (Stretch) with latest libreswan (IPsec VPN software) and xl2tpd (L2TP daemon).
+
+Docker images are built for quick deployment in various computing cloud providers.
+For more information on docker and containerization technologies, refer to [official document][1].
+
+## Prepare the host
+
+If you need to install docker by yourself, follow the [official installation guide][2].
+
+## Pull the image
+
+```bash
+$ docker pull teddysun/l2tp
+```
+
+This pulls the latest release of shadowsocks-libev.
+It can be found at [Docker Hub][3].
+
+## Start a container
+
+You **must create a environment file**  `/etc/l2tp.env` in host at first, and sample value is below:
+
+```
+VPN_IPSEC_PSK=teddysun.com
+VPN_USER=vpnuser
+VPN_PASSWORD=vpnpassword
+VPN_PUBLIC_IP=
+VPN_L2TP_NET=
+VPN_L2TP_LOCAL=
+VPN_L2TP_REMOTE=
+VPN_XAUTH_NET=
+VPN_XAUTH_REMOTE=
+VPN_DNS1=
+VPN_DNS2=
+```
+
+This will create a default user account for L2TP/IPsec VPN login, which can be used by your **multiple devices**. 
+The IPSec PSK (pre-shared key) is specified by the `VPN_IPSEC_PSK` environment variable. 
+The username is specified in `VPN_USER` environment variable.
+and password is specified in `VPN_PASSWORD` environment variable.
+If your VPS has multiple public IP addresses, maybe public IP need to specified in `VPN_PUBLIC_IP` environment variable.
+
+There is an example to start a container:
+
+```bash
+$ docker run -d --privileged -p 500:500/udp -p 4500:4500/udp --name l2tp --env-file /etc/l2tp.env -v /lib/modules:/lib/modules teddysun/l2tp
+```
+
+**Note**: The UDP port number `500` and `4500` must be opened in firewall.
+
+## Check container details
+
+If you want to view the container logs:
+
+```bash
+$ docker logs l2tp
+```
+
+Output log like below:
+
+```
+L2TP/IPsec VPN Server with the Username and Password is below:
+
+Server IP: Your Server public IP
+IPSec PSK: IPSec PSK (pre-shared key)
+Username : VPN username
+Password : VPN password
+
+Redirecting to: /etc/init.d/ipsec start
+Starting pluto IKE daemon for IPsec: Initializing NSS database
+
+xl2tpd[1]: Not looking for kernel SAref support.
+xl2tpd[1]: Using l2tp kernel support.
+xl2tpd[1]: xl2tpd version xl2tpd-1.3.12 started on 1d20eaecd9f2 PID:1
+xl2tpd[1]: Written by Mark Spencer, Copyright (C) 1998, Adtran, Inc.
+xl2tpd[1]: Forked by Scott Balmos and David Stipp, (C) 2001
+xl2tpd[1]: Inherited by Jeff McAdams, (C) 2002
+xl2tpd[1]: Forked again by Xelerance (www.xelerance.com) (C) 2006-2016
+xl2tpd[1]: Listening on IP address 0.0.0.0, port 1701
+```
+
+To check the status of your L2TP/IPSec VPN server, you can confirm `ipsec status` to your container like below:
+
+```bash
+$ docker exec -it l2tp ipsec status
+```
+
+## Manage VPN Users
+
+If you want to add, modify or remove user accounts, please do it simple like below:
+
+### List all users
+
+```bash
+$ docker exec -it l2tp l2tpctl -l
+```
+
+### Add a user
+
+```bash
+$ docker exec -it l2tp l2tpctl -a
+```
+
+### Delete a user
+
+```bash
+$ docker exec -it l2tp l2tpctl -d
+```
+
+### Modify a user password
+
+```bash
+$ docker exec -it l2tp l2tpctl -m
+```
+
+### Print help information
+
+```bash
+$ docker exec -it l2tp l2tpctl -h
+```
+
+
+[1]: https://docs.docker.com/
+[2]: https://docs.docker.com/install/
+[3]: https://hub.docker.com/r/teddysun/l2tp/