Flatcar distro support

[kartikjoshi21]

Background
Dalec already has a */testing/sysext route under some existing distro targets (e.g. azlinux3/testing/sysext, noble/testing/sysext).
However, these sysext images are currently produced “based on other distros” (Deb/RPM paths + workers), not via a Flatcar-native build environment.

Problem
Flatcar extensions are expected to be compatible with Flatcar’s userspace ABI and with Flatcar’s sysext conventions. Flatcar sysext images must follow specific naming + metadata rules:

1. Image should be named NAME.raw
2. Image must contain usr/lib/extension-release.d/extension-release.NAME
3. For Flatcar matching, ID=flatcar plus one of VERSION_ID or SYSEXT_LEVEL is required
   So “sysext output exists” is not sufficient; we need a dedicated Flatcar target and a Flatcar-native worker/toolchain.

Goals (Scope)

1. Add a dedicated build target for Flatcar
2. Build sysext images using a Flatcar-native build environment
3. Ensure produced sysext artifacts are directly usable on Flatcar:
4. Document usage for Flatcar

[invidian]

Dalec already supports building sysext, which is an official format for extending Flatcar installations. Does it satisfy your requirements?

[kartikjoshi21]

Dalec already supports building sysext, which is an official format for extending Flatcar installations. Does it satisfy your requirements?
@invidian Yes you are correct, sysext is definitely the right format, but I don’t think Dalec currently produces a Flatcar-consumable artifact out of the box. What I’m aiming for in this issue is making it a first-class flatcar target that emits a Flatcar-compatible sysext by default.
@cpuguy83 I think that's the next aim to enable flatcar support, right?

[invidian]

Oh right, because right now sysext images are built based on other distros:

$ make frontend && cat docs/examples/go-md2man.yml | docker buildx build --call targets --build-arg BUILDKIT_SYNTAX=local/dalec/frontend - | grep sysext
azlinux3/testing/sysext          Builds a systemd system extension image.
noble/testing/sysext             Builds a systemd system extension image.
trixie/testing/sysext            Builds a systemd system extension image.

And they should likely be built natively with Flatcar SDK or something, with dedicated target.

I'd update the issue description to include more requirements for it :)

[kartikjoshi21]

Oh right, because right now sysext images are built based on other distros:

$ make frontend && cat docs/examples/go-md2man.yml | docker buildx build --call targets --build-arg BUILDKIT_SYNTAX=local/dalec/frontend - | grep sysext
azlinux3/testing/sysext Builds a systemd system extension image.
noble/testing/sysext Builds a systemd system extension image.
trixie/testing/sysext Builds a systemd system extension image.
And they should likely be built natively with Flatcar SDK or something, with dedicated target.

I'd update the issue description to include more requirements for it :)

@invidian Thanks, i have updated the issue, i will update it more with more goals, I have started working on it.