package/docker-engine: introduce option to build CLI client statically

package/docker-engine: introduce option to build CLI client statically

This is useful when bind-mounting the docker client binary into
containers for a docker-in-docker setup.

The Docker CLI communicates with the daemon over a socket. It is
sometimes useful to bind-mount the Docker CLI and socket into containers at
run-time, so that scripts within the containers can call the Docker API
in the host using a client compatible with the host daemon.

Signed-off-by: Christian Stewart <christian@paral.in>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>