Also push insiders image tags on nightly and release

The insiders image variant is no longer built separately but some
consumers (e.g. ponylang/templates) still reference it. Tag and push
the regular image under the insiders name so those workflows keep
working.

Author: SeanTAllen

Makefile

@@ -17,9 +17,13 @@ push:
 
 push-nightly:
 	docker push "${IMAGE}:nightly"
+	docker tag "${IMAGE}:nightly" "${IMAGE}-insiders:nightly"
+	docker push "${IMAGE}-insiders:nightly"
 
 push-release:
 	docker push "${IMAGE}:release"
+	docker tag "${IMAGE}:release" "${IMAGE}-insiders:release"
+	docker push "${IMAGE}-insiders:release"
 
 pylint: build-nightly
 	docker run --entrypoint pylint --rm "${IMAGE}:nightly" /entrypoint.py