Add minimal docker documentation

Author: pascal-fb-martin

README.md

@@ -76,3 +76,7 @@ All these computers must be on the same subnet (UDP broadcast is involved for di
 
 The HouseSprinkler configuration is defined in a JSON file, by default /etc/house/sprinkler.json. However the proper way to configure is to access the Config page on the sprinkler's web UI. Do not forget to configure [houserelays](https://github.com/pascal-fb-martin/houserelays) first.
 
+## Docker
+
+The project supports a Docker container build, which was tested on an ARM board running Debian. To make it work, all the house containers should be run in host network mode (`--network host` option). This is because of the way [houseportal](https://github.com/pascal-fb-martin/houseportal) manages access to each service: using dynamically assigned ports does not mesh well with Docker's port mapping.
+