The "compat-volumes" setting shouldn't be provided by the compat build
endpoint, the docker daemon's isn't expected to recognize it.
It is also a conditional boolean flag, so if it isn't specified by the
caller, don't send a value from a client to the server, so that the
server will be able to apply its own default.
Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>