This changes the x86_64 image in nightly and final build steps to be prefixed
with "amd64-", and adds an arm64 image which is prefixed with "arm64-".
The unprefixed image, which was previously the x86_64 image, is now a
multi-architecture image which points to the two prefixed images.

This change also splits build_docker_image into build_docker_image_arm64
and build_docker_image_x86_64. The Build and Test Docker TeamCity builds
have been split accordingly, into arm64 and x86_64 variants for each.
These builds do not build/test the combined image, only the individual
platforms' images.

Part of CRDB-3143

Release note (general change): Cockroach Docker images are now multi
architecture manifests supporting the x86_64 (amd64) and arm64
architectures.