v0.8.2 Fix multi-source tilejson merge

What's Changed

• Fix merging multi-source tilejson by @nyurik in #653
  • Reported by @jtwiest in #652

ARM64 Docker Support

We keep trying to create an automated arm64 docker build, but it keeps failing.

It was first done by @Volatus in #613 (thank you!!!), but sadly we had to revert it -- the resulting docker image metadata was not being properly uploaded to the GitHub registry. If anyone has any experience in automating Docker builds for multiple platforms on GitHub, please let us know!

Full Changelog: v0.8.1...v0.8.2