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