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?