Created by: wangkuiyi

Without .dockerignore, docker build would send /build directory created by cmake as part of build context to Docker daemon. This makes the context as huge as 3GB and it takes a long while to sending.

$ docker build -t paddle:me -f paddle/scripts/docker/Dockerfile.cpu .
Sending build context to Docker daemon  2.95 GB

By setting .dockerignore with the same content as .gitignore, it is dozens of MB:

$ docker build -t paddle:me -f paddle/scripts/docker/Dockerfile.cpu .
Sending build context to Docker daemon 35.47 MB