feat: support composefs-native and plain ostree images in fisherman

[hanthor]

Overview

fisherman currently only supports standard OCI/bootc container images installed via bootc install to-filesystem. Add explicit support for:

1. composefs-native images — images using bootc's composefs backend (--experimental-composefs), where the filesystem is stored as a content-addressed composefs rather than a plain ostree commit
2. Plain ostree images — non-bootc OSTree repos (e.g. raw ostree pull + ostree admin deploy) for compatibility with non-bootc ostree-based distros (Fedora Silverblue pre-bootc, GNOME OS, etc.)

Motivation

• composefs is the future default backend for bootc and provides fs-verity image verification. Once it becomes the default, fisherman should handle it transparently.
• Plain ostree lets fisherman install non-bootc ostree images, widening compatibility significantly.
• Both are complementary to the existing unified storage work (--experimental-unified-storage).

composefs support

The composefs backend stores each file as a content-addressed object with fs-verity. Installation:

bootc install to-filesystem --experimental-composefs /target

Recipe field to add:

{ "backingStore": "composefs" }   // default: "ostree"

fisherman would pass --experimental-composefs to bootc install when set.

Note: composefs requires a filesystem that supports fs-verity (ext4, f2fs, btrfs). XFS does not currently support fs-verity — fisherman should validate this combination and reject xfs+composefs with a clear error.

ostree (non-bootc) support

For plain ostree repos, installation would use ostree CLI directly instead of bootc:

ostree pull-local /path/to/repo
ostree admin --sysroot=/target deploy REFSPEC

Or via remote:

ostree remote add origin https://...
ostree pull origin REFSPEC
ostree admin deploy origin:REFSPEC

Recipe additions needed:

{
  "imageType": "bootc",        // or "ostree"
  "ostreeRemote": "https://...",
  "ostreeRef": "fedora/40/x86_64/silverblue"
}

Related

• Unified storage issue: feat: Flatpak-packaged installer for installing any bootc image to disk or VM image #1
• bootc composefs docs: https://bootc-dev.github.io/bootc/composefs.html
• ostree admin deploy: https://ostreedev.github.io/ostree/man/ostree-admin-deploy.html
• fs-verity filesystem support: ext4 ✅, btrfs ✅, xfs ❌, f2fs ✅