When booting `docker-container` driver, it will pull and run image
`moby/buildkit:buildx-stable-1`.
If current node cannot connect to dockerhub, driver `docker-container`
will always booting failure.
But user may already load the image manually or pull it from a priviate
registry.
Buildx should check local docker image store after pull failed.
Fixes: #199 issuecomment-561996661
Signed-off-by: Lu Jingxiao <lujingxiao@huawei.com>
5b9d88b3ad