Adds service definition for AdGuard Home - master (1 of 2)

Adds service definition and documentation for AdGuard Home.

I did look, briefly, at the question of a "build.py" file but decided
that that question should be left for someone who understands if or
when it is necessary, and what is needed in it if it is. That someone
is not me. There are several templates that lack a "build.py"
(dashmachine, domoticz, heimdall, homer, portainer_agent, qbittorrent)
so it does not seem to be essential. It shows up in new menu OK. There
are no options that need to be configured in advance.

I made a conscious choice with port 8089 for the admin interface port
**because** it conflicts with PiHole. There are already going to be
other conflicts (eg 53) so this just rams home the point that you
can't run both.

Conversely, a search suggested no conflict with port 3001.

Author: Paraphraser

.templates/adguardhome/service.yml

@@ -0,0 +1,35 @@
+adguardhome:
+  container_name: adguardhome
+  image: adguard/adguardhome
+  restart: unless-stopped
+  environment:
+    - TZ=Etc/UTC
+  # enable host mode to activate DHCP server on ports 67/udp & 68/tcp+udp
+  # note that you must also disable all other ports if you enable host mode
+  # network_mode: host
+  ports:
+    # regular DNS
+    - "53:53/tcp"
+    - "53:53/udp"
+    # administration port (http)
+    # note: external and internal ports MUST be the same
+    #       not active until defined via setup port
+    - "8089:8089/tcp"
+    # HTTPS/DNS-over-HTTPS
+    # - "443:443/tcp"
+    # DNS-over-QUIC
+    # - "784:784/udp"
+    # DNS-over-TLS
+    # - "853:853/tcp"
+    # setup (http)
+    # note: only active until port 8089 becomes active
+    - "3001:3000/tcp"
+    # DNSCrypt
+    # - "5443:5443/tcp"
+    # - "5443:5443/udp"
+  volumes:
+     - ./volumes/adguardhome/workdir:/opt/adguardhome/work
+     - ./volumes/adguardhome/confdir:/opt/adguardhome/conf
+  networks:
+    - iotstack_nw
+    - vpn_nw

docs/Containers/AdGuardHome.md

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+# AdGuard Home
+
+## References
+
+* [AdGuard Home GitHub](https://github.com/AdguardTeam/AdGuardHome)
+* [AdGuard Home DockerHub](https://hub.docker.com/r/adguard/adguardhome)
+
+## Either *AdGuard Home* or *PiHole*, but not both
+
+AdGuard Home and PiHole perform similar functions. They use the same ports so you can **not** run both at the same time. You must choose one or the other.
+
+## <a name="quickStart"> Quick Start </a>
+
+When you first install AdGuard Home:
+
+1. Use a web browser to connect to it using port 3001. For example:
+
+	```
+	http://raspberrypi.local:3001
+	```
+
+2. Click "Getting Started".
+
+3. Change the port number for the Admin Web Interface to be "8089". Leave the other settings on the page at their defaults and click "Next".
+4. Enter a username and password and click "Next".
+5. Click "Open Dashboard". This redirects to port 8089.
+6. After the initial setup, you connect to AdGuard Home via port 8089:
+
+	```
+	http://raspberrypi.local:8089
+	```
+
+## About port 8089
+
+Port 8089 is the default administrative user interface for AdGuard Home running under IOTstack.
+
+Port 8089 is not active until you have completed the [Quick Start](#quickStart) procedure. You must start by connecting to port 3001.
+
+Because of AdGuard Home limitations, you must take special precautions if you decide to change to a different port number:
+
+1. The internal and external ports **must** be the same; and
+
+2. You **must** convince AdGuard Home that it is a first-time installation: 
+
+	```
+	 $ cd ~/IOTstack
+	 $ docker-compose stop adguardhome
+	 $ docker-compose rm -f adguardhome
+	 $ sudo rm -rf ./volumes/adguardhome
+	 $ docker-compose up -d adguardhome
+	```
+
+3. Repeat the [Quick Start](#quickStart) procedure, this time substituting the new Admin Web Interface port where you see "8089".
+
+## About port 3001:3000
+
+Port 3001 (external, 3000 internal) is only used during [Quick Start](#quickStart) procedure. Once port 8089 becomes active, port 3001 ceases to be active.
+
+In other words, you need to keep port 3001 reserved even though it is only ever used to set up port 8089.
+
+## About Host Mode
+
+If you want to run AdGuard Home as your DHCP server, you need to put the container into "host mode". You need edit the AdGuard Home service definition in `docker-compose.yml` to:
+
+1. add the line:
+
+	```
+	network_mode: host
+	```
+
+2. remove the `ports:` directive and **all** of the port mappings.
+
+Note:
+
+* It is not really a good idea to offer DHCP services from a container. This is because containers generally start far too late in a boot process to be useful. If you want to use AdGuard Home for DHCP, you should probably consider a native installation.