Cages should start with cached packages already installed

[DemiMarie]

How to file a helpful issue

The problem you're addressing (if any)

Even with the Qubes executor, installing the cached packages is often the (subjectively) slowest part of a build.

The solution you'd like

Cached packages (fetched with init-cache) should be pre-installed into a chroot, avoiding the need to install the packages every time. The local and container executors can bind-mount the chroot into the cage read-only for even more speed wins, and the Qubes executor might be able to use a Plan 9 filesystem mount.

This should not be done for CI builds (and possibly production builds) because it hides missing build dependencies. That’s not an issue for developer builds, though.

The value to a user, and who that user might be

All contributors will benefit from faster builds.

Completion criteria checklist

(This section is for developer use only. Please do not modify it.)

[marmarek]

Similar to the other ticket (and as you noted in the description), this should be opt-in.

[marmarek]

And BTW, this one should be quite easy to implement, contrary to #9702 .

[DemiMarie]

Similar to the other ticket (and as you noted in the description), this should be opt-in.

Should it? It definitely needs to be configurable, but I expect that most developers will have it on while CI and production builds have it off. If this does not have a security trade-off, I would expect to have the default be on, with CI and production explicitly disabling the feature. If a developer messes up a dependency, CI will catch the problem.