Information on installing docker prereqs

Signed-off-by: g2flyer <[email protected]>

docs/docker.md

@@ -3,5 +3,15 @@ Licensed under Creative Commons Attribution 4.0 International License
 https://creativecommons.org/licenses/by/4.0/
 --->
 
-See the [PDO Docker README](../docker/README.md).
+To develop using docker, you will have to install a recent version of
+the `docker` daemon, _including build-kit_, and you also need
+`docker-compose`. On ubuntu, the easiest way is to achieve that is to
+run `sudo apt install docker docker-buildx docker-compose`. For other
+platforms or more recent versions, see the [official docker
+site](https://docs.docker.com/engine/install/).   See also the
+official documentation for any [post-install
+steps](https://docs.docker.com/engine/install/linux-postinstall/),
+e.g., configuring it with systemd and/or to work with a proxy
+
+For usage of docker, see the [PDO Docker README](../docker/README.md).