Add Docker build and possibly Docker Image test to Travis build

[justb4]

After a push to master the Travis build may succeed but the then triggered Docker build on DockerHub may fail.

Add a Docker build to Travis YAML build (Travis supports Docker as a service) and possibly even test the Docker Image as it contains test config+data. This could be effected via:

docker run -it geopython/pygeoapi test (on the Docker Image built within Travis off course..)

[tomkralidis]

@justb4 status on this one? I think we are building and running Docker in Travis?

[justb4]

Closing: now have GH Workflow Docker build/push and new issue #1003