✨ Improved garbage collection defaults

[jedevc]

Spinning this out of a private discord discussion here.

We inherit buildkit's default gc policies - which is to try and keep cache usage under 10% of available disk.

This is not actually great for a lot of cases. Here's a few dagger scenarios, and what kind of cache policies we might want for these:

• Using dagger on a personal linux machine
  • I want dagger to use quite a lot of space - I use dagger for most of my dev work, so maybe it could use 20% of my disk (on the scale of several hundred gigs of data).
• Using dagger in docker-desktop/lima/etc on a personal mac/windows/linux machine
  • Because this is a VM, there's a disk provisioned for it - that's usually quite small (the docker desktop seems unreasonably so sometimes).
  • I want dagger to use most of the available disk storage (but not all), maybe something like 50% (this works out to mostly the same as the above option - but now there are two dials to turn, the docker desktop disk size dial, and the dagger disk size dial).
• Using dagger in a dedicated CI worker
  • I want dagger to use as much storage as it requires - I want the limit to be something like 80-90%.

At the moment, we just have a flat "10%" for everything - which is pretty bad, especially for the last scenario.

Part of this is related to documentation + usability of configuring cache policy in the first place:

• Document how to configure local cache size #7022
• ✨ dagger cli should have cache subcommands #6260

However, this is still different - we should have a way of making sure that the defaults are good for 90% of users. There's unfortunately no easy way to detect what kind of case we're in.

I think this makes sense more sense in the context of #5583 though. When we start a dagger engine with a driver, we can allow specifying the cache policies for that engine, and potentially automatically pick some reasonable defaults (e.g. based on the presence of GITHUB_ env vars, etc).