Fix custom root path feature

[feedmeapples]

What was changed

Fixes custom root path feature TEMPORAL_WEB_ROOT_PATH in prod build.
Client static files will now be placed under the specified subpath in dist folder:

/usr/app # cd dist/
/usr/app/dist # ls custom-path-example/*
custom-path-example/favicon.png                            custom-path-example/temporal.c502a579d6c1648a6205.css.map
custom-path-example/index.html                             custom-path-example/temporal.c502a579d6c1648a6205.js
custom-path-example/logo-rounded.png                       custom-path-example/temporal.c502a579d6c1648a6205.js.map
custom-path-example/temporal.c502a579d6c1648a6205.css
/usr/app/dist # 

Serving the static files from server/index.js will also include the subpath

Why?

Fixes the feature

Checklist

1. Closes issue:

2. How was this tested:

3. Any docs updates needed?