alpine armhf base image with qemu, enable capability build armhf image on x86 and docker hub
https://github.com/resin-io-projects/armv7hf-debian-qemu
https://resin.io/blog/building-arm-containers-on-any-x86-machine-even-dockerhub/
3.3
3.4
3.5
3.6, latest
edge
RUN [ "cross-build-start" ]
...
RUN [ "cross-build-end" ]