Feature tracking bug for building ChromeOS images inside Docker on Swarming.

ChromeOS infra is exploring the possibility of running builds (including binary CrOS image generation) inside Docker containers as part of CI on LUCI Swarming. This has a lot of advantages to us and reduces our overall infra overhead. For this to be a tenable goal, Swarming bots would need to support caching/GCing (potentially large) Docker images on local disk.

Currently collected requirements are as:
- The Docker daemon should be used for all Docker cache maintenance, trying to handle caching via /var/lib/docker files would either not work or be a mess.
 - A client library for Docker should be used for this
- Only one large image would need to be cached at any one point in time (around 30GB)
 - This image would be specific to CrOS builds (needs bot/build affinity)
 - This image would be a :latest on gcr.io, updates there should result in:
  - A `docker pull` from gcr.io (Docker does this for you by default)
  - A flush of the old image from cache (this can be lazy; it can be part of the Swarming GC)
 - Several other ephemeral images could be created / push during a build that need to be flushed reliably
 - Containers need to be cleaned up (100% of them can be nuked between runs, as long as the image cache persists)
- Several Docker commands should be supported from Recipe including (but not limited to):
 - `docker build` from within a workspace with a Dockerfile
 - `docker run` with the privileged flag and volume mounting like `-v /dev:/dev` (used for kernel EXT4 image mounting from within the container)
 - `docker commit` for committing a container to an image
 - `docker push` for pushing images to gcr.io

Experiments done (currently ACL restricted, ping athilenius for access, sorry random project was used for this):
- CrOS has been fully built inside a Docker container (including image gen)
 - cros_sdk image (the large one that needs to be cached): gcr.io/the-em-drive/doccros_cros_sdk
 - fully_build_image (this image would only be pushed to enable local debugging on workstations in the event of a failure): gcr.io/the-em-drive/doccros_build_image
- These steps were replicated in isolation on Cloud Build:
 - depo_tools checkout -> fully built cros_sdk: https://pantheon.corp.google.com/cloud-build/builds/228cc47d-2023-4054-aa5e-6befe45b222c?project=the-em-drive&organizationId=433637338589
 - setup_boad -> build_image: https://pantheon.corp.google.com/cloud-build/builds/a4850b34-62a7-4d71-a214-3e7d8898da0e?project=the-em-drive&organizationId=433637338589

Other refs:
crbug.com/808836 - Swarming bot: Design containment API
crbug.com/764493 - Use cgroups in Swarming task