The bitwarden_rs code is still cross-compiled exactly as before, but Docker
Buildx is used to rewrite the resulting Docker images with correct platform
metadata (reflecting the target platform instead of the build platform).
Buildx also now handles building and pushing the multi-arch manifest lists.
With some apt/dpkg magic building multidb containers for arm versions
now also works. As long as the build stage and docker-image stage use
the same base (debian buster now) it should all work.
Resolves #530, resolves #1066
