feat(CI/CD): Build arm64 container images

[levaitamas]

Motivated by #75, this PR adds support for building both arm64 and am64 container images with the workflows.

Note: dev tags already have arm64 images:

• https://hub.docker.com/r/l7mp/stunnerd/tags
• https://hub.docker.com/r/l7mp/stunner-gateway-operator/tags