add DockerHub link to downloads page

Author: tomkralidis

docs/download.md

@@ -21,3 +21,7 @@ title: Download
 #### UbuntuGIS
 
 pygeoapi exists as a UbuntuGIS package in [Launchpad](https://launchpad.net/~ubuntugis/+archive/ubuntu/ubuntugis-unstable/+packages?field.name_filter=pygeoapi&field.status_filter=published&field.series_filter=)
+
+### Docker
+
+* [DockerHub](https://hub.docker.com/r/geopython/pygeoapi)