feat: apt upgrade on rebuild + dated Docker Hub tag

[DeepDiver1975]

What

1. apt-get upgrade -y on rebuild — both v22.04/Dockerfile.multiarch and v24.04/Dockerfile.multiarch previously ran only apt-get update + install, so rebuilds stayed pinned to the base image's original package set. Adding apt-get upgrade -y pulls in the latest package versions (including security fixes) on every rebuild.

2. Immutable dated Docker Hub tag — adds an opt-in build-date-tag boolean input to the reusable docker-build.yml, enabled from main.yml. On each publish, an additional <docker-tag>-<YYYYMMDD> (UTC) tag is pushed alongside the existing floating tags.

Result

Per matrix version, on merge to master and the weekly cron rebuild, Docker Hub receives:

• owncloud/ubuntu:22.04 / owncloud/ubuntu:24.04 — unchanged floating tags
• owncloud/ubuntu:22.04-<YYYYMMDD> / owncloud/ubuntu:24.04-<YYYYMMDD> — new immutable dated tags

This gives traceability: you can tell which dated build a container came from and pin to a specific historical build.

Notes

• The date tag is version-prefixed so the two matrix builds never collide on the same day.
• build-date-tag defaults to false, so other consumers of the reusable workflow are unaffected.
• The build-native self-test job is untouched (push: false, never publishes).
• Tag-logic dry-run verified output: owncloud/ubuntu:22.04 + owncloud/ubuntu:22.04-<date>.

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