Example for building multi-arch Docker images in parallel

[fkorotkov]

Description

Since now docker_builder supports multiple architectures, it is theoretically possible to build a multi-arch images on two separate VMs in parallel and enjoy native performance, then we can just create a combined manifest similar to how they do it in this blog post.

This way we can drastically improve build times for Cirrus Agent and Cirrus CLI.

Context

Multi-arch builds are too slow because of QEMU emulation