addresses documentation problems raised in issue 35 (#36)

Responses to #35:

1. Revises `docker run` example:

	* places container in host mode
	* runs container detached (so it does not appear to stall)
	* passes the user and group IDs for the local user (so persistent
	  storage is owned by the local user)
	* changes default mechanism for net-filter rules to `iptables-nft`
	* places image reference on a separate line

2. Revises `docker compose` service definition:

	* removes deprecated `version` clause, substituting the
	  "here comes YAML" `---` signature
	* changes default mechanism for net-filter rules to `iptables-nft`

3. Re-writes explanation of `ZEROTIER_ONE_USE_IPTABLES_NFT` to make it
   clear that `true` is usually the correct option for all implementations
   (not just Raspberry Pi). Also adds explicit test to confirm that the
   container's net-filters are getting into the host's tables.

Opportunistic changes:

1. Updates URLs for ZeroTier Knowledge Base article (avoids redirects
   when following those links).

2. Clarifies that it is the ZeroTier Knowledge Base article which
   implements a half-router, and summarises the additional capabilities
   of `zerotier-router`.

3. Adds cross-reference to IOTstack documentation (IMO this is more
   comprehensive than anything in the ZeroTier Knowledge Base).

4. Removes extraneous `0x09` characters.

Fixes #35

Signed-off-by: Phill Kelley <34226495+Paraphraser@users.noreply.github.com>

Author: Paraphraser

README-router.md

@@ -4,24 +4,32 @@
 
 This is a variation built on top of the zyclonite/zerotier container which implements a local network router. It is based upon the ZeroTier Knowledge Base article:
 
-* [Route between ZeroTier and Physical Networks](https://zerotier.atlassian.net/wiki/spaces/SD/pages/224395274/Route+between+ZeroTier+and+Physical+Networks)
+* [Route between ZeroTier and Physical Networks](https://docs.zerotier.com/route-between-phys-and-virt/)
 
-Technically, this could be described as a *half-router*:
+Technically, the above approach could be described as a *half-router*:
 
 * You can initiate connections *from* a remote client *to* devices on the LAN; but
 * You can't initiate connections *to* the remote client *from* devices on the LAN.
 
+This implementation extends the concept so that you have a choice of:
+
+* Permitting remote clients to initiate connections with a devices on your LAN; or
+* Permitting clients on your LAN to initiate connections with remote devices reachable across your ZeroTier Cloud network; or
+* Both of the above (ie a full router).
+
 ### Command line example
 
 ``` console
 $ docker run --name zerotier-one --device=/dev/net/tun \
+  --network=host -d \
   --cap-add=NET_ADMIN --cap-add=NET_RAW --cap-add=SYS_ADMIN \
-  --env TZ=Etc/UTC --env PUID=999 --env PGID=994 \
+  --env TZ=Etc/UTC --env PUID=$(id -u) --env PGID=$(id -g) \
   --env ZEROTIER_ONE_LOCAL_PHYS=eth0 \
-  --env ZEROTIER_ONE_USE_IPTABLES_NFT=false \
+  --env ZEROTIER_ONE_USE_IPTABLES_NFT=true \
   --env ZEROTIER_ONE_GATEWAY_MODE=inbound \
   --env ZEROTIER_ONE_NETWORK_IDS=«yourDefaultNetworkID(s)» \
-  -v /var/lib/zerotier-one:/var/lib/zerotier-one zyclonite/zerotier:router
+  -v /var/lib/zerotier-one:/var/lib/zerotier-one \
+  zyclonite/zerotier:router
 ```
 
 Note:
@@ -35,7 +43,8 @@ Note:
 ### Compose file example
 
 ``` yaml
-version: '3'
+---
+
 services:
   zerotier:
     image: "zyclonite/zerotier:router"
@@ -55,7 +64,7 @@ services:
       - PUID=999
       - PGID=994
       - ZEROTIER_ONE_LOCAL_PHYS=eth0
-      - ZEROTIER_ONE_USE_IPTABLES_NFT=false
+      - ZEROTIER_ONE_USE_IPTABLES_NFT=true
       - ZEROTIER_ONE_GATEWAY_MODE=inbound
     # - ZEROTIER_ONE_NETWORK_IDS=«yourDefaultNetworkID(s)»
 ```
@@ -87,9 +96,9 @@ Note:
 	- PUID=1000
 	- PGID=1000
 	```
-	
+
 	If omitted, `PUID` defaults to user ID 999, while `PGID` defaults to group ID 994.
-	
+
 	These variables are only used to ensure consistent ownership of persistent storage on each launch. They do not affect how the container *runs.* Absent a `user:` directive, the container runs as root and does not downgrade its privileges.
 
 * `ZEROTIER_ONE_LOCAL_PHYS` - a space-separated list of physical interfaces that should be configured to participate in NAT-based routing. Examples:
@@ -107,31 +116,45 @@ Note:
 		environment:
 		- ZEROTIER_ONE_LOCAL_PHYS=wlan0
 		```
-	
+
 	- If your computer has both Ethernet and WiFi interfaces active and you wish to be able to route through each interface:
-	
+
 		``` yaml
 		environment:
 		- ZEROTIER_ONE_LOCAL_PHYS=eth0 wlan0
 		```
-		
+
 		This scheme could be appropriate where the physical interfaces were:
-		
+
 		1. In the same broadcast domain (subnet). Disconnecting Ethernet would fail-over to WiFi.
 		2. In different broadcast domains, such as if you allocated different subnets for Ethernet and WiFi.
 
-* `ZEROTIER_ONE_USE_IPTABLES_NFT` - controls the command the container uses to set up NAT forwarding. Example:
+* `ZEROTIER_ONE_USE_IPTABLES_NFT` - controls the command the container uses to set up net-filter rules to implement packet forwarding. Example:
 
-	``` yaml
-	environment:
-	- ZEROTIER_ONE_USE_IPTABLES_NFT=true
 	```
-	
-	- `false` means the container uses `iptables`. This is the default.
-	- `true` means the container uses `iptables-nft`.
+	 environment:
+	 - ZEROTIER_ONE_USE_IPTABLES_NFT=true
+	```
+
+	* `false` means the container uses `iptables-legacy`. This is the default if the variable is omitted but that is only to maintain backwards compatibility.
+	* `true` means the container uses `iptables-nft`. This is *generally* what you need.
+
+	The way to be absolutely certain is to start the container and then run the following command:
+
+	``` console
+	$ sudo nft list ruleset | grep -c "zt*"
+	```
+
+	Ignore any lines that start with the `#` character.
+
+	There are three possible responses:
+
+	1. An error saying that the `nft` command has not been found. Docker uses `iptables-nft` to construct its own net-filter rules so it installs the `iptables` package as a dependency which, in turn, installs `nftables` as its own dependency. For that reason, not being able to find the `nft` command generally indicates an improper installation of Docker.
+	2. A line-count of zero. This means the container has not been able to configure net-filter rules on the host. If that happens, try the opposite setting for this environment variable (eg `true` instead of `false`).
+	3. A non-zero line-count. That means the container has been able to propagate net-filter rules into the host's tables, which is what you want. The actual number is not important, just something other than zero.
+
+	The container will always come up. Once you've authorised the client in ZeroTier Central, it will be able to join your ZeroTier Cloud network. Tests like `ping` and `traceroute` that you run on the same host will always work. However, if the container is not able to propagate its net-filter rules into the host's tables, traffic *beyond* the host where the container is running will not work properly. The problem is quite subtle so it's always a good idea to check that the host has the expected net-filters.
 
-	Try `true` if NAT does not seem to be working. This is needed on Raspberry Pi Bullseye.
-	
 * `ZEROTIER_ONE_GATEWAY_MODE` - controls the traffic direction. Examples:
 
 	- Only permit traffic *from* the ZeroTier cloud *to* the local physical interfaces:
@@ -140,7 +163,7 @@ Note:
 		environment:
 		- ZEROTIER_ONE_GATEWAY_MODE=inbound
 		```
-		
+
 	- Only permit traffic *from* the local physical interfaces *to* the ZeroTier cloud:
 
 		``` yaml
@@ -156,18 +179,18 @@ Note:
 		```
 
 	Defaults to `inbound` if omitted. Note that you will probably need one or more static routes configured in your local LAN router so that traffic originating in a local host which is not running the ZeroTier client can be directed to the gateway host.
-	
+
 * `ZEROTIER_ONE_NETWORK_IDS` – a space-separated list of ZeroTier network IDs.
 
 	This variable is *only* effective on first launch. There is no default if it is omitted. Examples:
-	
+
 	- to join a single network:
 
 		``` yaml
 		environment:
 		- ZEROTIER_ONE_NETWORK_IDS=aaaaaaaaaaaaaaaa
 		```
-		
+
 		Equivalent of running the following command after the container first starts:
 
 		```
@@ -194,6 +217,15 @@ Note:
 
 For each ZeroTier container that is configured as a router, ZeroTier needs at least one *Managed Route*.
 
-The [ZeroTier Wiki](https://zerotier.atlassian.net/wiki/spaces/SD/pages/224395274/Route+between+ZeroTier+and+Physical+Networks#Configure-the-ZeroTier-managed-route) explains how to design managed routes.
+The [ZeroTier Wiki](https://docs.zerotier.com/route-between-phys-and-virt/#configure-the-zerotier-managed-route) explains how to design managed routes.
 
 You configure Managed Routes in ZeroTier Central.
+
+### Detailed examples
+
+See:
+
+* [SensorsIot/IOTstack - ZeroTier](https://sensorsiot.github.io/IOTstack/Containers/ZeroTier/)
+
+You do not have to use IOTstack just to get ZeroTier running. However, the IOTstack documentation explores several network models and is a useful guide to the concepts involved and decisions you will need to make.
+