Create docker.rst

Providing docker-compose example in the doc following #1334

docs/docker.rst

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+.. _docker:
+
+Docker
+======
+
+Glances can be installed through Docker, allowing you to run it without installing all the python dependencies directly on your system. Once you have [docker installed](https://docs.docker.com/install/), you can
+
+Get the Glances container:
+
+.. code-block:: console
+
+    docker pull nicolargo/glances
+
+Run the container in *console mode*:
+
+.. code-block:: console
+
+    docker run --rm -v /var/run/docker.sock:/var/run/docker.sock:ro --pid host --network host -it docker.io/nicolargo/glances
+
+Additionally, if you want to use your own glances.conf file, you can create your own Dockerfile:
+
+.. code-block:: console
+
+    FROM nicolargo/glances
+    COPY glances.conf /glances/conf/glances.conf
+    CMD python -m glances -C /glances/conf/glances.conf $GLANCES_OPT
+
+Alternatively, you can specify something along the same lines with docker run options:
+
+.. code-block:: console
+
+    docker run -v ./glances.conf:/glances/conf/glances.conf -v /var/run/docker.sock:/var/run/docker.sock:ro --pid host -it docker.io/nicolargo/glances
+
+Where ./glances.conf is a local directory containing your glances.conf file.
+
+Run the container in *Web server mode* (notice the `GLANCES_OPT` environment variable setting parameters for the glances startup command):
+
+.. code-block:: console
+
+    docker run -d --restart="always" -p 61208-61209:61208-61209 -e GLANCES_OPT="-w" -v /var/run/docker.sock:/var/run/docker.sock:ro --pid host docker.io/nicolargo/glances
+
+You can also include Glances container in you own `docker-compose.yml`. Here's a realistic example including a "traefik" reverse proxy serving an "whoami" app container plus a Glances container, providing a simple and efficient monitoring webui.
+    
+.. code-block:: console
+
+    version: '3'
+
+    services:
+      reverse-proxy:
+        image: traefik:alpine
+        command: --api --docker
+        ports:
+          - "80:80"     
+          - "8080:8080"
+        volumes:
+          - /var/run/docker.sock:/var/run/docker.sock
+
+      whoami:
+        image: emilevauge/whoami
+        labels:
+          - "traefik.frontend.rule=Host:whoami.docker.localhost"
+
+      monitoring:
+        image: nicolargo/glances:latest-alpine
+        restart: always
+        pid: host
+        volumes:    
+          - /var/run/docker.sock:/var/run/docker.sock
+        environment:
+          - "GLANCES_OPT=-w"
+        labels:      
+          - "traefik.port=61208"      
+          - "traefik.frontend.rule=Host:glances.docker.localhost"
+