Build multi-arch docker image

[DazWorrall]

Hey 👋 I wonder if you would accept this pull request to build the docker image on both linux/arm64 and linux/amd64? I've tested these workflows on my fork, they behave the same on merge (tagging with branch-short_sha) and pushing a tag (tagging with the version) as they do today, and I confirmed that I can start containers on both architectures from the built image.

The changes look more extensive than they are, to make this work there is a bit more setup and a switch to the docker/build-push-action action, which supports buildx+qemu.

[DazWorrall]

@Dev25 hey 👋 any feedback on this PR?

[Dev25]

Thanks for the PR @DazWorrall, i'll review this weekend.

[Dev25]

LGTM, thanks for the PR!

[Dev25]

One unintended consequence of this PR was the ghcr hosted package is now ghcr.io/dev25/mcrouter_exporter instead of the nested ghcr.io/dev25/mcrouter_exporter/mcrouter_exporter.

Most users are still pulling from quay.io instead of GH, so i'm fine with fixing the GH image path although i will look at pushing arm64 images to quay.io via github actions (and get rid of legacy CircleCI builds) next week.

Can cut a 0.4.0 release in the mean time if you need @DazWorrall or wait till quay.io is also updated.

[DazWorrall]

One unintended consequence of this PR was the ghcr hosted package is now ghcr.io/dev25/mcrouter_exporter instead of the nested ghcr.io/dev25/mcrouter_exporter/mcrouter_exporter.

Whoops, sorry, I missed that the package name was doubled up.

Can cut a 0.4.0 release in the mean time if you need @DazWorrall or wait till quay.io is also updated.

No rush for me, I can pull straight from GitHub for my needs. Thanks very much for accepting the PR 🙏