403 on apt.dockerproject.org

General
1

Permissions on at least part of the APT repos is broken:

Err:19 https://apt.dockerproject.org/repo/pool xenial/main amd64 Packages
  403  Forbidden
W: The repository 'https://apt.dockerproject.org/repo/pool xenial Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch https://apt.dockerproject.org/repo/pool/dists/xenial/main/binary-amd64/Packages  403  Forbidden
E: Some index files failed to download. They have been ignored, or old ones used instead.

Same problem with testing:

Err:18 https://apt.dockerproject.org/repo/pool xenial/testing amd64 Packages   
  403  Forbidden
W: The repository 'https://apt.dockerproject.org/repo/pool xenial Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch https://apt.dockerproject.org/repo/pool/dists/xenial/testing/binary-amd64/Packages  403  Forbidden
E: Some index files failed to download. They have been ignored, or old ones used instead.
2

Found the workaround near the bottom of this thread:

In your /etc/apt/sources.list (or equivalent), replace ‘xenial’ with ‘ubuntu-xenial’:

$ grep xenial /etc/apt/sources.list.d/additional-repositories.list 
deb https://apt.dockerproject.org/repo/ ubuntu-xenial main
#deb https://apt.dockerproject.org/repo/pool/ xenial testing