Multiarch implementation for amd64, arm64 and arm32

[CodeCutterUK]

As promised in #518 here is a potential change to support multiple architectures.

I couldn't see a next branch as per contribution guidelines so based this off of master instead.

[CodeCutterUK]

I might be able to change this to store the individual builds with Travis until the manifest is deployed, which would get around needing the Docker username and password, but will await initial feedback

[kingdonb]

I would like to take some time to review this, but unfortunately don't have access to any ARM hosts or other than AMD64 right now, for at least several months. Is there someone who can vouch for this who has some of the architectures in question?

[Theaxiom]

@CodeCutterUK yeah sorry, contribution guidelines need to be updated -- we don't have next branch anymore as it is simplified with trunk-based development (feature/task-based branching) flow. We really appreciate the contribution, I am tempted to just merge it in and then fail forward if it doesn't work. What do you think @kingdonb ?

[Theaxiom]

LGTM! 👍

[CodeCutterUK]

Happy to fix forward....only thing to be aware of is that I can't test the Docker Hub publication parts, so might be worth double checking the publish to latest & master parts work as expected and don't overwrite anything.

[Theaxiom]

Changes have been merged in.