Relates to [cli@27b2797], which forked this type from the Moby API, and
[moby@6cfff7e], which made the same change on the API side.
The Email field was originally used to create a new Docker Hub account
through the `docker login` command. The `docker login` command could be
used both to log in to an existing account (providing only username and
password), or to create a new account (providing desired username and
password, and an e-mail address to use for the new account).
This functionality was confusing, because it was implemented when Docker
Hub was the only registry, but the same functionality could not be used
for other registries. This functionality was removed in Docker 1.11 (API
version 1.23) through [moby@aee260d], which also removed the Email field
([engine-api@9a9e468]) as it was no longer used.
However, this caused issues when using a new CLI connecting with an old
daemon, as the field would no longer be serialized, and the deprecation
may not yet be picked up by custom registries, so [engine-api@167efc7]
added the field back, deprecated it, and added an "omitempty". There
was no official "deprecated" format yet at the time, so let's make sure
the deprecation follows the proper format to make sure it gets noticed.
[cli@27b2797]: 27b2797f7d
[moby@6cfff7e]: 6cfff7e880
[moby@aee260d]: aee260d4eb
[engine-api@9a9e468]: 9a9e468f50
[engine-api@167efc7]: 167efc72bb
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
aab947de8f