Add multiarch build to Actions

[mocdaniel]

With more and more people relying on ARM architectures at home and for work, I thought it might be a good idea to provide multiarch images of this great webhook provider.

I tested the resulting ARM build in my environment successfully already.

[rberrelleza]

@mocdaniel do you know if we need to update Chart.yaml to signal that this is supported on ARM clusters?

[mocdaniel]

@mocdaniel do you know if we need to update Chart.yaml to signal that this is supported on ARM clusters?

As per the official docs on the Helm file, we don't need to update anything in that regard. One of the few (😉) pain points of Helm charts...