Add support for multi platform builds

[brpaz]

Context

ARM-based platforms are getting more and more popular, and most well know projects and helm charts already support ARM architectures. (Ex: Prometheus, Grafana, Postgres, Cert Manager, etc). The only thing that I need in my Kubernetes cluster that was missing support for ARM was this operator.

This PR aims to fix that. Closes #123

Changes

• Dockerfile - Add support for multi-platform builds following recommendations from this and this. Also refactored a bit the Dockerfile with cache support, which is important to optimize the build time, when building for multiple platforms.
• GitHub Actions - Refactored the release workflow to leverage, official docker GitHub actions, which makes building images for multi-platform easier. Reference: https://docs.docker.com/build/ci/github-actions/multi-platform/

[hitman99]

Great PR, thank you for the contribution