Build arm64 container in runner instead of QEMU

[badlop]

This PR builds the arm64 container image using a native arm machine, instead of using QEMU inside an amd64 machine. That possibility is available in GitHub since a few months ago.

Now both images are built in parallel and take just 5 minutes, instead of the 30 minutes required to build arm64 in QEMU.

The only problem is that now the images for each architecture (amd64 and arm64) must be built in a different runner, obviously, and when both jobs have finished, the container manifest must be rebuilt. In practice this ends up in a large and duplicated container.yml, but I couldn't find a more clean way to achieve this.

[coveralls]

coverage: 33.841%. remained the same
when pulling 8e5b86e on badlop:arm_runner
into fb572bf on processone:master.