Publish multi-arch image

[douglascamata]

All the Observatorium repos are currently publishing only x86_64 images.

This makes is much more difficult for folks with arm64 computers (like Apple Silicon Macbooks) to be able to contribute to the projects, as most of them use the efficientgo/e2e framework for e2e tests based on container images.

We should publish multi-arch images with at least x86_64 and arm64 architectures. Other architectures could be added in the future.

Proposal

• Development container builds should be possible without any cross-build tooling.

• Only release builds should be using a cross-build tool like Docker's buildx or podman (its build command supports building multi-arch images).

• The compilation process should take advantage of the Go's amazing cross-compilation features and avoid compiling under a different architecture's QEMU. Docker has a cool article on how to do this.

Projects to migrate to multi-arch

• observatorium/api (done at Prepare for multi-arch image build with GH Actions api#536)
• observatorium/up (done at Build & publish multi arch image up#46)
• observatorium/thanos-receive-controller (done at Add support for multi-arch images with docker buildx thanos-receive-controller#98)
• observatorium/token-refresher (done at Add multi arch image build token-refresher#20)
• observatorium/rules-objstore (done at Add support for multi-arch images with docker buildx rules-objstore#12)
• observatorium/thanos-rule-syncer (done at Release multi arch images thanos-rule-syncer#15)
• observatorium/opa-ams
• observatorium/opa-openshift

They all have a similar approach to how container images and built and published. After one of them is migrated to multi-arch the steps to migrate the others will be mostly the same.

[douglascamata]

I'll try to slowly tackle this starting by the token-refresher, because it has the less probability of being disruptive to development of other features.