Adjust proxy_pass for registry to force IPv4

What does this MR do?

In some instances localhost can resolve to ::1 and by default the registry backend listens on 127.0.0.1 only.

This changes the default setting from localhost:5000 to 127.0.0.1:5000 to force IPv4 to prevent this happening.

Updated the existing tests (I believe) - shouldn't be any need for additional ones.

Related issues

Internal ZD Issue - customer reported intermittent issues and logs showed seemingly random attempts to connect back via v6. Hard to replicate given the seemingly random choice of the resolver but this change should force things to v4.

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened