Fix slim integration for multiarch builds in GitHub CI

[lreiher]

• Integrate docker-slim for waaaaaaaay smaller run images #33 has integrated built-in support for slim. A hotfix is required to run slim in GitHub jobs that build multiarch images, i.e., platforms: amd64,arm64 and enable-singlearch-push: false.
• Currently, the lines responsible for pushing the slimmed image cannot handle multiarch-tags, see this mqtt_client action log.
• The fix is to re-run slim in the second push execution of ci.sh, where platforms are not treated separately anymore. A current shortcoming is that slim is then only executed for the native platform, i.e., amd64 in most cases. You would end up with latest (multiarch) and latest-slim (singlearch).
• A successful mqtt_client action log is found here.