Fix and improve adding-extensions docs

Not sure what you mean with The ".phpunit-base" in current docs is wrong in general case.

There are two examples provided in the docs, which come from actual modules:

You can use apt-get to install new packages. For example, imagemagick needs extra packages:

.phpunit-base:
  before_script:
    - apt-get update
    - apt-get install -y --no-install-recommends imagemagick

Or Redis needs the redis PHP extension:

.phpunit-base:
  before_script:
    - apt-get update
    - apt-get install -y --no-install-recommends $PHPIZE_DEPS
    - pecl install redis && docker-php-ext-enable redis

So it shows how to add a package and an extension, viewing the basic apt-get, pecl and docker-php-ext-enable usage.

I'm not sure that we can cover all cases, but if you add an example here we can definitely add it to the docs.

Yep, redis needs the extension *in the phpunit job*, but ymmv...
My prob was different and now sessionless may serve as another example to link to.
But i don't want to spread more half-baked knowledge, please let people who know better write that doc.

Did the code you added here work? https://git.drupalcode.org/project/sessionless/-/blob/3.x/.gitlab-ci.yml...

That, together with the other examples, would help us improve this section of the documentation, but I want to make sure that it's working before assuming anything.