Trim dependencies

[jnoordsij]

This PR aims to trim dependencies to reduce image size and build time. The efforts are based on the discussion here and subsequent changes in composer/composer#12580. From what I've been able to deduce myself and based on the discussions, all of this can be dropped without any impact (or otherwise very minimal impact).

I've deliberately only opted to target the latest image and it may even be worth considering doing this only from 2.10 forwards, to even further reduce the potential scope of impact.

Note that dropping mercurial could also be considered to heavily reduce image size, being responsible for most of the image size (in comparison to the PHP base image), however given that I see it still being valid for some usecases, keeping it around makes some sense.

[alcohol]

But to what end? Image size is not really a concern for the images that are not binary-only. If end users have a real need for a lean image, they can make their own.

[jnoordsij]

Yeah fair, the benefits are very marginal, it just saves a slight bit of build time and network load for dependencies that seem to be entirely obsolete nowadays from what I can gather. And moreover it also would serve (at least it did for me) as a bit of an example which tooling might be relevant when only using the binary in your own image.

But I do admit there's no certainty it will not break anything, so if some parts or all of this seems to superfluous, I can also fully understand not bothering with this further.