Builds linux/amd64 + linux/arm64 images via Buildx/QEMU whenever a "v*"
tag is pushed or a release is published, and pushes them (tagged with
the version and "latest") to this repository's own container registry.
Written in plain GitHub Actions syntax, which Gitea Actions runs as-is.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
