Comandos que corrí:
Buildear las 2 imagenes
$ docker build -t pablokbs/multiarch-helloworld:armhf armhf
$ docker build -t pablokbs/multiarch-helloworld:amd64 amd64
Pushear las 2 imagenes
$ docker push pablokbs/multiarch-helloworld:armhf
$ docker push pablokbs/multiarch-helloworld:amd64
Crear el manifest
$ docker manifest create pablokbs/multiarch-helloworld:latest \
pablokbs/multiarch-helloworld:armhf \
pablokbs/multiarch-helloworld:amd64
Docker manifest push
$ docker manifest push pablokbs/multiarch-helloworld:latest