我在拉取镜像时，总是下载不了，报这样的错。请问是什么原因？如何解决？
$ sudo docker pull jupyter/jupyterhub
Using default tag: latest
latest: Pulling from jupyter/jupyterhub
3be020ef5d53: Pulling fs layer
a6ae39d5de27: Pulling fs layer
53edc248c850: Pulling fs layer
2ec411e8581b: Pulling fs layer
ff5a9885bb1c: Pulling fs layer
58ff83677668: Pulling fs layer
4011f9cc84dd: Pulling fs layer
c5c48ff79ffc: Pulling fs layer
72d94d2ef49a: Pulling fs layer
9913dffcd1e2: Pulling fs layer
211309e80840: Pulling fs layer
860aac654fb0: Pulling fs layer
4c1b642ceda5: Pulling fs layer
3be020ef5d53: Pull complete
5a7e779541c9: Download complete
673860eb3fcb: Download complete
cccda61854ad: Download complete
b3b1404070b6: Download complete
dab4341d6221: Download complete
c96992c66b17: Download complete
a3bd3751109d: Verifying Checksum
f89d3b450eac: Download complete
6d63004b6886: Download complete
7e1ad4f59ea4: Download complete
fd5445f17a96: Download complete
d3a1f33e8a5a: Already exists
c22013c84729: Already exists
d74508fb6632: Already exists
91e54dfb1179: Already exists
ff6a52bb5dff: Already exists
65f6189d96c0: Already exists
2bdbdc94f5f9: Already exists
172a4d23f0db: Already exists
e89f5d34b67d: Already exists
90ba00f3f3ac: Already exists
d5ae19dc314b: Already exists
eb52461ba975: Already exists
Pulling repository docker.io/jupyter/jupyterhub
Error while pulling image: Get https://index.docker.io/v1/repositories/jupyter/jupyterhub/images: dial tcp: lookup index.docker.io: no such host
