Customizing UI adding new item in navigation

If I want to change UI and add new item in navigation bar then are there any changes required to do in Gemfile.
Another question is do I need to create own plugin to achieve this.
And how to update docker image so that I can check whether new code changes are working or not.

I think you will have to write your own provider to achieve this. (I haven’t done anything like this yet)
There is this blog post from januar, that talks about something similar

Regarding the docker image: You probably want to switch to a full blown developer setup