Create CI component for multi-arch image builds using Docker buildx
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## Problem
We're currently missing a [CI template](https://docs.gitlab.com/ee/development/cicd/templates.html) that can be used to demonstrate how to build multi-arch Docker images using [buildx](https://docs.docker.com/build/architecture/).
## Proposal
Add a new component to the GitLab.com CI/CD component catalog that illustrates how to leverage GitLab CI to build multi-arch images. As a plus, we can hint at the possibility of using one job per target architecture, likely relying on [build matrixes](https://docs.gitlab.com/ee/ci/yaml/#parallelmatrix).
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