Move index.html to simplify .gitlab-ci.yml

## What does this MR do?

- move `index.html` to public folder
- simplify `.gitlab-ci.yml`

## Are there points in the code the reviewer needs to double check?

Not really...

## Why was this MR needed?

Every build moves the `index.html` to the public folder—which is unnecessary if it can be there from the beginning.

## What are the relevant issue numbers?

None.


See merge request !10

Use smaller Docker image to speed up build

See merge request !9