This patch adds a new builder subcommand, allowing to add more builder-related
commands in the future. Unfortunately `build` expects an argument so could not
be used as a subcommand.
This also implements `docker builder prune`, which is needed to prune the builder
cache manually without having to call `docker system prune`.
Today when relying on the legacy builder, users are able to prune dangling images
(used as build cache) by running `docker image prune`. This patch allows the
same usecase with buildkit.
Signed-off-by: Tibor Vass <tibor@docker.com>
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