Add an info field for features like buildkit

[sudo-bmitch]

Signed-off-by: Brandon Mitchell git@bmitch.net

- What I did

Added a client feature section to docker info to expose whether buildkit is enabled. I'm not certain this should be a map of strings, if we want an array like Plugins, hardcode a struct, or some other way to track. This would fix moby/moby#42350

- How I did it

This is using the same call from docker build --help to adjust the help message for arguments on whether buildkit is enabled.

- How to verify it

$ docker info --format '{{index .ClientInfo.Features "buildkit"}}'
enabled

$ DOCKER_BUILDKIT=0 docker info --format '{{index .ClientInfo.Features "buildkit"}}'

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[AkihiroSuda]

Can this be just []string?

[sudo-bmitch]

Yes, that's something I'm very open to. I think there are several options and I'm happy to go with any:

1. Features map[string]string with each feature able to take a value, this could be something other than enabled for other features.
2. Features []string and just have each feature a name. Simple and too the point.
3. Features []feature where feature is just type feature struct { name string } today, but able to add more fields if needed in the future. This mirrors the plugin layout.
4. Buildkit bool just hardcode a single field for buildkit, and then need another for the next feature later. This has the advantage of being easy to template with --format.