Docker image points to old garethr/kubernetes-json-schema repo

Using the current-latest `garethr/kubeval` Docker image (at the moment `0.14.0`) doesn't work with newer schemas from [instrumenta/kubernetes-json-schema](https://github.com/instrumenta/kubernetes-json-schema).

From what I gather from https://github.com/instrumenta/kubeval/issues/135, that is the "official" Docker image for kubeval.

Based on https://github.com/garethr/kubernetes-json-schema/issues/26, and from finding `https://kubernetesjsonschema.dev/` in the source, it appears that instrumenta/kubernetes-json-schema should be the place to read schemas from.

For example, using the [v1.15.7 schema](https://github.com/instrumenta/kubernetes-json-schema/tree/74551ef8/v1.15.7-standalone-strict) doesn't currently work:
```
$ docker run --rm -t -v `pwd`:`pwd`:ro -w `pwd` garethr/kubeval:0.14.0 \
--strict -v 1.15.7 *.yml

1 error occurred:

* Problem loading schema from the network at
https://raw.githubusercontent.com/garethr/kubernetes-json-schema/master/v1.15.7-standalone-strict/deployment.json:
Could not read schema from HTTP, response status is 404 Not Found
```