docs: Add a "migrating from rpm-ostree" doc

[cgwalters]

Let's document here how we expect one would perform a migration from a system currently using (rpm-)ostree to one using bootc.

[chapmanjacobd]

I'm interested in helping out with this!

My reason for wanting to switch over is that I have a Fedora 42 system that I don't have physical access to which has a weird issue but only when installing ffmpeg (I think I ctrl+c'd during an upgrade once):

error: Writing rpmdb: Failed to update rpmdb (rpmtsRun code -1)

sudo rpm-ostree install ffmpeg
Checking out tree d2b718a... done
Enabled rpm-md repositories: fedora-cisco-openh264 updates fedora rpmfusion-free-updates rpmfusion-free rpmfusion-nonfree-updates rpmfusion-nonfree google-chrome charm
Updating metadata for 'updates'... done
Importing rpm-md... done
rpm-md repo 'fedora-cisco-openh264' (cached); generated: 2024-08-21T16:04:02Z solvables: 3
rpm-md repo 'updates'; generated: 2025-06-03T05:13:17Z solvables: 15776
rpm-md repo 'fedora' (cached); generated: 2025-04-09T11:06:59Z solvables: 76879
rpm-md repo 'rpmfusion-free-updates' (cached); generated: 2025-05-30T08:25:20Z solvables: 47
rpm-md repo 'rpmfusion-free' (cached); generated: 2025-04-12T09:12:27Z solvables: 358
rpm-md repo 'rpmfusion-nonfree-updates' (cached); generated: 2025-05-30T09:13:38Z solvables: 27
rpm-md repo 'rpmfusion-nonfree' (cached); generated: 2025-04-12T09:32:45Z solvables: 218
rpm-md repo 'google-chrome' (cached); generated: 2025-06-03T22:18:33Z solvables: 4
rpm-md repo 'charm' (cached); generated: 2025-05-30T13:45:50Z solvables: 562Resolving dependencies... done
Will download: 150 packages (107.1?MB)
Downloading from 'fedora'... done
Downloading from 'fedora-cisco-openh264'... done
Downloading from 'rpmfusion-free'... done
Downloading from 'rpmfusion-free-updates'... done
Downloading from 'updates'... done
Importing packages... done
Checking out packages... done
Running systemd-sysusers... done
Running pre scripts... done
Running post scripts... done
Running posttrans scripts... done
Writing rpmdb... done
error: Writing rpmdb: Failed to update rpmdb (rpmtsRun code -1)

I can install ffmpeg somewhat successfully (dnf gets to 150/152 before Transaction failed: Rpm transaction failed. but ffmpeg is fully useable for my situation) with sudo rpm-ostree usroverlay --hotfix and dnf. So I'd like to start over with bootc--but it's not really obvious how to do so.

rpm-ostree status

State: idle
AutomaticUpdates: apply; rpm-ostreed-automatic.timer: last run 5h 15min ago Deployments:
* fedora-iot:fedora/stable/x86_64/iot
Version: 42.20250603.0 (2025-06-03T11:12:45Z)

BaseCommit: d2b718ad7c555e882a03f02b28f27c4ea1261888c7590cdcc88320bd1e9a302f
GPGSignature: Valid signature by B0F4950458F69E1150C6C5EDC8AC4916105EF944
LayeredPackages: acpi bfs btrfsmaintenance cargo dnscrypt-proxy earlyoom expect
fatrace fd-find fish flatpak fontconfig git git-delta git-lfs
google-noto-color-emoji-fonts gum hddtemp htop ImageMagick
inotify-tools kitty-terminfo libacl-devel libattr-devel
libcap-devel libpq-devel lm_sensors lshw lsof mkvtoolnix
moreutils nano ncdu nmon oniguruma-devel parallel pg_top
postgresql-devel progress pspg python python-devel python3-pip
qbittorrent-nox ripgrep rpmfusion-free-release
rpmfusion-nonfree-release rsync sqlite-devel syncthing tailscale
tlp tlp-rdw trash-cli unar which zoxide
Unlocked: hotfix

Previously, I rebased from Kinoite to IoT and that went relatively smoothly:

sudo rpm-ostree rebase fedora-iot:fedora/stable/x86_64/iot

I'd like to keep my current /etc and /home data.

I guess the existing RHEL documentation is not far off...

I just remembered that I previously put together a small bootc demo but I am not 100% confident about whether bootc upgrade will work exactly the same as rpm-ostree rebase: https://github.com/chapmanjacobd/computer/tree/main/.github%2Fbootc%2Fserver