[#49133237] Correct domain for assets proxy_pass
[#49133237] Correct domain for assets proxy_pass

The `proxy_pass` should refer to the current domain (e.g.
`production.alphagov.co.uk`) rather than always being preview.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet