ci(github-actions): Build multi-arch images for nightly builds

[SISheogorath]

This patch enabled multi-arch builds for nightly builds, this should help to deploy hedgedoc on devices like a Raspberry Pi without resorting to third-party images.

Initially, this should be done by nightly images, to evaluate if this works properly. It can later-on be enabled on the general releases as well.

[ErikMichelson]

Do we want to add support for linux/arm/v7 as well? While the Raspberry Pi OS added support for arm64 (Pi 3 + 4) recently, most people probably still use the 32 bit builds.

[davidmehren]

Obsoleted by https://github.com/hedgedoc/container/pull/398