Unable to build worker-app image from voting example

Docker
#1

root@ip-10-16-0-131:~/example-voting-app/worker# docker build . -t worker-app
[+] Building 0.6s (4/4) FINISHED
=> [internal] load build definition from Dockerfile 0.1s
=> => transferring dockerfile: 252B 0.0s
=> [internal] load .dockerignore 0.1s
=> => transferring context: 2B 0.0s
=> ERROR [internal] load metadata for docker.io/microsoft/dotnet:2.0.0-sdk 0.4s
=> [auth] microsoft/dotnet:pull token for registry-1.docker.io 0.0s

[internal] load metadata for docker.io/microsoft/dotnet:2.0.0-sdk:

Dockerfile:1

1 | >>> FROM microsoft/dotnet:2.0.0-sdk
2 |
3 | WORKDIR /code

ERROR: failed to solve: microsoft/dotnet:2.0.0-sdk: pull access denied, repository does not exist or may require authorization: server message: insufficient_scope: authorization failed
root@ip-10-16-0-131:~/example-voting-app/worker#

#2

Please check this error message.