Pass the HTTP host header to webpack-dev-server

What does this MR do?

Passes the HTTP Host: header to the webpack-dev-server process when proxying requests for frontend assets.

This one-liner was split off of !46459 (merged) because merging this separately will help prevent situations where switching between old branches could break webpack and cause confusing errors for gdk users.

e.g.

1. User runs gdk start in a HEAD without this change
2. User switches to a branch in which webpack.config.js has disableHostCheck: false
3. User encounters 500 errors on all frontend assets until they run gdk restart rails-web

The plan is to merge this, then merge !46459 (merged) in a week or so to avoid the above scenario.

Screenshots (strongly suggested)

n/a

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