Make email optional for Credentials

[Alan Fung]

The docker login client on our server does not support -e and our private registry does not take email anyway.

Expected:

Save Docker task with Email field omitted.

Actual:

Docker task fails to save with the message You must specify a email.

[Alexey Chystoprudov]

Tracked here - https://jira.atlassian.com/browse/BAM-18613

[Alexey Chystoprudov]

fixed in Bamboo 6.2.0