ci: docker: cross-platform build, test and deploy

• docker:build: build docker image directly on amd64 and arm64 architecture
• docker:build:cross-platform: build cross-platform docker image for linux/amd64, linux/arm64/v8 and linux/arm/v7 pushed to registry.nic.cz/knot/knot-resolver/cross-platform
  • only for tags and master branch
• docker:test:cross-platform:run some basic tests on a built cross-platform docker image.
• dockerhub:deploy: deploy cross-platform image from registry.nic.cz/knot/knot-resolver/cross-platform to DockerHub cznic/knot-resolver
  • only for tags