Add unix to allowed hosts

What does this MR do?

In Rails 6, the new HostAuthorization middleware is enabled by default (it's not enabled on other environments). It currently allows localhost and IP addresses and we add Gitlab.config.gitlab.host as additional allowed host.

When gitlab-shell makes a request via unix socket, it uses http://unix/<endpoint> so the host will be unix. Since it's not included in the list of allowed hosts, it gets blocked.

This adds unix host to the list of allowed hosts.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #199701 (closed)