Add support for reverse proxy subpaths

[davide-malagoli-ict-group-it]

Problem I wanted to publish the dashboard as a subpath of a domain using a reverse proxy, but I wasn't able to make it work because of <base href="/" />

Solution Many application nowadays support the use of an environment variable that changes the base href of the entire application, assets included

Additional context Another possibility would be using relative URLs, this way the addition of the subpath would become seamless for the app

[bigkevmcd]

@davide-malagoli-ict-group-it Does https://docs.gitops.weave.works/docs/guides/run-ui-subpath/ help at all?