Add container-storage support for Podman and Buildah (#5368)

* feat: add containers-storage support for Podman and Buildah, update tests and documentation

Signed-off-by: Bruce Clark <bruce.clark@mandg.com>

* test: update containers-storage tests to validate package.json and handle missing images

Signed-off-by: Bruce Clark <bruce.clark@mandg.com>

* test: fix rootless containers-storage test setup

Install uidmap for Podman and Buildah CI jobs.
Align temporary rootful and rootless storage paths.
Clean up rootless stores within the builder's user namespace.

Signed-off-by: Bruce Clark <bruce.clark@mandg.com>

* fix: only use containers-storage when explicitly requested

- containers-storage is no longer part of automatic image resolution; it's used only for `--from containers-storage`, the `containers-storage:` scheme, or `default-pull-source: containers-storage` (now an accepted value). Opening a store runs full graph driver init (mounts, locks, store writes), which is too invasive as a probe on every plain image reference.
- rootless stores must be read from inside the builder's user namespace (`podman unshare syft ...`), documented in the README.
- the `containers_image_openpgp` tag is now applied to unit/integration tests and lint, so dev tooling compiles the same provider that ships.
- containers-storage CI runs through `make containers-storage-test` and covers both vfs and overlay stores.

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

* fix: exclude cgo btrfs driver and unmount overlay store in cleanup

- cgo-enabled linux builds (`-race` tests, lint) compile go.podman.io's btrfs graph driver, which needs libbtrfs headers. Add `exclude_graphdriver_btrfs` alongside `containers_image_openpgp` everywhere the tag is set; release builds are `CGO_ENABLED=0` and never included it.
- the overlay driver bind-mounts its graphroot inside the rootless user namespace, so the containers-storage test unmounts it before removing the store, and cleanup no longer fails the run.

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

---------

Signed-off-by: Bruce Clark <bruce.clark@mandg.com>
Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>
Co-authored-by: Bruce Clark <bruce.clark@mandg.com>
Co-authored-by: Alex Goodman <wagoodman@users.noreply.github.com>

.github/workflows/release.yaml

@@ -46,7 +46,7 @@ jobs:
       # these are checks that should be run on pull-request and merges to main.
       # we do NOT want to kick off a release if these have not been verified on main.
       # Please see the validations.yaml workflow for the names that should be used here.
-      checks: '["Acceptance tests (Linux)", "Acceptance tests (Mac)", "Build snapshot artifacts", "CLI tests (Linux)", "Integration tests", "Static analysis", "Unit tests"]'
+      checks: '["Acceptance tests (Linux)", "Acceptance tests (Mac)", "Build snapshot artifacts", "CLI tests (Linux)", "Containers-storage tests (Podman)", "Containers-storage tests (Buildah)", "Integration tests", "Static analysis", "Unit tests"]'
 
   release:
     needs: [check-gate, version-available]