derivations are left on disk
derivations can be fetched by hash and name
outputs can be fetched, this is safe because the derivation contains the system name

If we pack something into a docker container we are limited to just during a derivation into a docker container.

Steps for gc:

take all of our derivations that we have tagged as output derivations
enumerate their output dependencies, check which output dependencies belong to which input derivations
all derivation files and their sources get to stay, delete all outputs we don't depend on
delete all recently downloaded files?

keep derivation and sources on the filesystem, that way you can always rebuild the outputs
remove all derivations that link to nothing
by keeping the derivations and their sources we will always have a cache hit on starlark parsing, and if an output is needed at a later point we can recalculate it

Description of algorithm

We're interested in a subset of derivation fields:

// Derivation is the basic building block of a Bramble build
type Derivation struct {
	Outputs          map[string]Output
	InputDerivations InputDerivations
	InputSource      InputSource
}

take a derivation we want to keep (is it for runtime or for build input?)
scan all of the outputs, keep all of the derivations that are used as outputs
keep all of the input sources
loop through the inputDerivations and process all of them, keeping them for runtime if they are linked to the output dependencies