Use multi-platform testimages

[gesellix]

The docker manifest command is available (with "experimental":"enabled") since Docker 18.02-ce: docker/cli#138

Inspiration for a multi platform matrix build on travis-ci: https://stefanscherer.github.io/cross-build-nodejs-with-docker/

[gesellix]

still needs some automation:

docker run --rm mplatform/mquery gesellix/testimage:os-windows
Image: gesellix/testimage:os-windows
 * Manifest List: No
 * Supports: amd64/windows

docker run --rm mplatform/mquery gesellix/testimage:os-linux
Image: gesellix/testimage:os-linux
 * Manifest List: No
 * Supports: amd64/linux

docker manifest create gesellix/testimage gesellix/testimage:os-linux gesellix/testimage:os-windows
docker manifest push gesellix/testimage

docker run --rm mplatform/mquery gesellix/testimage
Image: gesellix/testimage
 * Manifest List: Yes
 * Supported platforms:
   - linux/amd64
   - windows/amd64:10.0.14393.447

[gesellix]

See https://github.com/docker-client/echo-server for a multi-platform image.