[online] Honor GENERATOR_HOST env variable

[jimschubert]

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• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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Description of the PR

This sets "host" appropriately to the published host when GENERATOR_HOST
is configured.

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[jimschubert]

@OpenAPITools/generator-core-team just FYI. Encountered this after adding Rapidoc, it looks like it was an issue before this as well but we were only forwarding to the api-doc JSON.

[jimschubert]

Found and issue, fixed in #3265

[cbornet]

Springfox already has a springfox.documentation.swagger.v2.host to do that.
Otherwise it can detect the hostname from the request (respecting the x-forwarding-* headers) so forcing it to localhost by default is not correct.