Build docker image for CI

CPU image

docker build -t dgl-cpu -f Dockerfile.ci_cpu .

GPU image

docker build -t dgl-gpu -f Dockerfile.ci_gpu .

Lint image

docker build -t dgl-lint -f Dockerfile.ci_lint .

CPU MXNet image

docker build -t dgl-mxnet-cpu -f Dockerfile.ci_cpu_mxnet .

GPU MXNet image

docker build -t dgl-mxnet-gpu -f Dockerfile.ci_gpu_mxnet .