Homu retry log - libc

Time (UTC) PR Message
2021-05-11 15:31:19 2157
@bors retry
2021-05-11 15:22:45 2157
@bors retry
2021-05-11 15:22:29 2167
@bors retry
2021-05-11 15:15:44 2157
@bors retry
2021-05-07 06:25:25 2157
@bors retry
2021-04-04 13:57:21 2136
```
 Step 1/7 : FROM ubuntu:20.04
Head registry-1.docker.io/v2/library/ubuntu/manifests/20.04: Get auth.docker.io/token?account=githubactions&scope=repository%3Alibrary%2Fubuntu%3Apull&service=registry.docker.io: EOF
```

I remember this failure happened on rust-lang/rust yesterday, docker.io is fragile recently... @bors retry