This allows all subcommands to use this flag.
Additionally reads the default value for the flag from the
`BUILDX_BUILDER` env var.
Precedence is:
CLI ARG > flag > env var > config file
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
Currently the user can type `docker buildx` to use this tool.
This patch allows the user to install buildx as a `docker builder` alias.
As an additional benefit, this allows the regular `docker build` to hook
into `buildx build`.
Note that the install and uninstall commands are currently hidden.
Signed-off-by: Tibor Vass <tibor@docker.com>
