clarify copyright in Dockerfile

FYI here at the FOSS4G 2019 code sprint, we are working to make GHC an OSGeo Community project. As part of the [review](https://lists.osgeo.org/pipermail/incubator/2019-August/003940.html), the following was asked for clarification:

https://github.com/geopython/GeoHealthCheck/blob/master/Dockerfile#L6

(note the original PR was in #40).

@yjacolin what did you want to use as a license for your contributions? Note that GHC is MIT as a rule, but looking for your input/clarification.