Change initContainer code to manipulate path for config.js

[artntek]

Our attempt to use a relative path for config.js was unsuccessful - see #2454

The only viable alternative at this stage is to edit this line in index.html when installign metacatui:

    var appConfigPath = "/config/config.js";

Since this file is also used for non-k8s deployments, we can't add go templating.

We already have an initContainer defined in deployment.yaml, so it should be pretty straightforward to add code there in order to inser the root path before the beginning of the appConfigPath shown above.