Add hadolint (or another Dockerfile linter) to the extensions.json

[leograba]

This is a feature request.

I am guiding a new intern that don't know much about Docker (much like several people coming from traditional non-container Linux, or Windows). I noticed he doing some things that are not best practices on the Dockerfile.

I just saw that hadolint has an extension (https://marketplace.visualstudio.com/items?itemName=exiasr.hadolint) and though that it would be nice if new projects created with our extension recommended to install this extension here.

I'm not sure if this must be done at template level, or extension level, sorry if this is not the right place.