Add docker image build for arm/v6 and arm/v7

[aBMania]

Hi,

I'm trying to run signatory on raspberry pi, which use arm/v6 or arm/v7 architecture. (depending on the RPi version).

I've building it manually, but having it on the signatory docker hub repository would be nice.

Thanks, Sylvain

[stephengaudet]

https://github.com/ecadlabs/signatory/pull/251 we got armv7 with PR 251

if we add armv6 we can close this issue