Windows buildkite agents are bundling shared state into per-agent qcow2 images

[staticfloat]

We currently have a two-layer setup in our Windows KVM setup; a base image that gets built once and per-agent customized images. Theoretically, the base image should be something that we want for any kind of windows work we want to do (and therefore shouldn't have buildkite-specific stuff there) but we're installing an awful lot of stuff into the buildkite-agent-specific images (such as pulling docker image layers to heat up the cache drive so that docker pull is faster in the future).

We should either:

• Add another layer, the base image -> buildkite-specific shared state -> agent-specific state.
• Incorporate more buildkite-agent-specific stuff into the base image.

Either way, we will significantly reduce the amount of state in the agent-specific qcow2 images, which will save disk space, speed up rebuild time, and reduce reboot time, as a significant limiting factor in our agent restart time is how long it takes to copy over a new pristine agent image. (The base image and the proposed new middle image would both be read-only and shared among multiple agents, of course)