Docs: Give a little example about how to use extensions with Docker

[joselcvarela]

Scope

We recommend to use Directus with Docker.
Though I didn't find any example of developing extensions within Docker.

In this PR we show a very simple way of how to develop custom extensions while using Docker

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[paescuj]

I think this is a bit of a clash with https://docs.directus.io/extensions/installing-extensions.html. IMO if we want to keep both guides, we need to make it clearer that this new one is for developing, while the other one is for production.