Preserve flatpak repo cache across (local user) build reruns to speed them up

[vjr]

What Happened?

The liveiso subdirectory build mkosi.conf runs PrepareScripts=./iso-flatpaks.chroot which works in a chroot environment because, well, the script name ends in .chroot.

The flatpak repo remotes (appcenter and flathub) cache ends up inside this chroot and is lost (ephemeral) when the overall build is rerun even without a prior just clean intending an incremental build.

Observe the apt packages cache is preserved because there is no chroot in that build stage, so, for example, if a fresh build (first time or after a just clean) completes in say 15 minutes then any subsequent builds without doing just clean takes maybe half the time to complete depending on the speed of your computer and internet. You can see the tangible differences in the apt install and flatpak install logs during the builds.

Steps to Reproduce

1. Rerun just do-daily immediately after a successful build.
2. Observe that while the rebuild duration reduces by a lot due to the apt package cache being preserved the flatpak install logs indicate fresh installs re-downloading everything.

Expected Behavior

Speedup "incremental" rebuilds even m0ar by preserving the flatpak cache across reruns.

Image Version

Current main build system.

Install Type

Atomic

Hardware Platform

64-bit AMD/Intel (x86)

Log Output

Hardware Info

No response

[vjr]

I've been trying (and so far failing) to address this issue so if someone can give me advice on how to solve it I'd be grateful.

I've mainly tried mucking about to avoid/delay the chroot and/or expose/relocate the podman mounts outside the chroot so the flatpak cache is preserved with no luck, while trying the set the FLATPAK_SYSTEM_CACHE_DIR env var mentioned at https://docs.flatpak.org/en/latest/flatpak-command-reference.html

I just saw that I have not tried FLATPAK_DOWNLOAD_TMPDIR yet.

[vjr]

This has also been handy: https://github.com/systemd/mkosi/blob/main/mkosi/resources/man/mkosi.1.md