I can reproduce this bug on my Mac which should aid in confirmation for `armhf`.

```
Edwards-MacBook-Air:~ emv$ docker run --rm armhf/ubuntu:xenial apt-get update

Unable to find image 'armhf/ubuntu:xenial' locally
xenial: Pulling from armhf/ubuntu
3ec2010dcf9a: Pull complete 
a9ce7d83bdeb: Pull complete 
a096d24b8544: Pull complete 
e1d64facf085: Pull complete 
6300ac39294b: Pull complete 
Digest: sha256:ad289f5b5c2a0bd88f61333b0d6f0ea81051c6f315a8eb22de0907e1da960115
Status: Downloaded newer image for armhf/ubuntu:xenial
Get:1 http://ports.ubuntu.com/ubuntu-ports xenial InRelease [247 kB]
Get:2 http://security.ubuntu.com/ubuntu xenial-security InRelease [102 kB]
Get:3 http://ports.ubuntu.com/ubuntu-ports xenial-updates InRelease [102 kB]
Get:4 http://ports.ubuntu.com/ubuntu-ports xenial-backports InRelease [102 kB]
Get:5 http://security.ubuntu.com/ubuntu xenial-security/universe Sources [30.0 kB]
Ign:6 http://security.ubuntu.com/ubuntu xenial-security/main armhf Packages
Ign:7 http://security.ubuntu.com/ubuntu xenial-security/restricted armhf Packages
Ign:8 http://security.ubuntu.com/ubuntu xenial-security/universe armhf Packages
Ign:9 http://security.ubuntu.com/ubuntu xenial-security/multiverse armhf Packages
Ign:6 http://security.ubuntu.com/ubuntu xenial-security/main armhf Packages
Ign:7 http://security.ubuntu.com/ubuntu xenial-security/restricted armhf Packages
Ign:8 http://security.ubuntu.com/ubuntu xenial-security/universe armhf Packages
Ign:9 http://security.ubuntu.com/ubuntu xenial-security/multiverse armhf Packages
Ign:6 http://security.ubuntu.com/ubuntu xenial-security/main armhf Packages
Ign:7 http://security.ubuntu.com/ubuntu xenial-security/restricted armhf Packages
Ign:8 http://security.ubuntu.com/ubuntu xenial-security/universe armhf Packages
Ign:9 http://security.ubuntu.com/ubuntu xenial-security/multiverse armhf Packages
Err:6 http://security.ubuntu.com/ubuntu xenial-security/main armhf Packages
  404  Not Found [IP: 91.189.88.161 80]
Ign:7 http://security.ubuntu.com/ubuntu xenial-security/restricted armhf Packages
Ign:8 http://security.ubuntu.com/ubuntu xenial-security/universe armhf Packages
Ign:9 http://security.ubuntu.com/ubuntu xenial-security/multiverse armhf Packages
Get:10 http://ports.ubuntu.com/ubuntu-ports xenial/universe Sources [9802 kB]
Get:11 http://ports.ubuntu.com/ubuntu-ports xenial/main armhf Packages [1486 kB]
Get:12 http://ports.ubuntu.com/ubuntu-ports xenial/restricted armhf Packages [8491 B]
Get:13 http://ports.ubuntu.com/ubuntu-ports xenial/universe armhf Packages [9531 kB]
Get:14 http://ports.ubuntu.com/ubuntu-ports xenial/multiverse armhf Packages [149 kB]
Get:15 http://ports.ubuntu.com/ubuntu-ports xenial-updates/universe Sources [185 kB]
Get:16 http://ports.ubuntu.com/ubuntu-ports xenial-updates/main armhf Packages [592 kB]
Get:17 http://ports.ubuntu.com/ubuntu-ports xenial-updates/restricted armhf Packages [8116 B]
Get:18 http://ports.ubuntu.com/ubuntu-ports xenial-updates/universe armhf Packages [517 kB]
Get:19 http://ports.ubuntu.com/ubuntu-ports xenial-updates/multiverse armhf Packages [5194 B]
Get:20 http://ports.ubuntu.com/ubuntu-ports xenial-backports/main armhf Packages [4914 B]
Get:21 http://ports.ubuntu.com/ubuntu-ports xenial-backports/universe armhf Packages [2561 B]
Fetched 22.9 MB in 5min 15s (72.5 kB/s)
Reading package lists...
E: Failed to fetch http://security.ubuntu.com/ubuntu/dists/xenial-security/main/binary-armhf/Packages  404  Not Found [IP: 91.189.88.161 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.
```