install/flat: Use composefs pipeline for rootfs and related changes

- Use composefs-rs for SELinux labeling and kernel installation
- Implement streaming copy for kernel/initramfs files
- Consolidate bootloader installation logic
- Add /usr overlay status tracking
- Various test and documentation updates

Note: blockdev.rs and ADOPTERS.md changes moved to separate PRs per review feedback

Assisted-by: Claude Code (Opus)
Signed-off-by: Eric Curtin <eric.curtin@docker.com>

Author: ericcurtin

Cargo.toml

@@ -44,10 +44,11 @@ clap_mangen = { version = "0.2.20" }
 # If adding/removing crates here, also update docs/Dockerfile.mdbook and docs/src/internals.md.
 #
 # To develop against a local composefs-rs checkout, add a [patch] section at the end of this file:
-#   [patch."https://github.com/containers/composefs-rs"]
+#   [patch."https://github.com/composefs/composefs-rs"]
 #   composefs = { path = "/home/user/src/composefs-rs/crates/composefs" }
 #   composefs-boot = { path = "/home/user/src/composefs-rs/crates/composefs-boot" }
 #   composefs-oci = { path = "/home/user/src/composefs-rs/crates/composefs-oci" }
+#   cfsctl = { path = "/home/user/src/composefs-rs/crates/cfsctl" }
 # The Justfile will auto-detect these and bind-mount them into container builds.
 composefs = { git = "https://github.com/composefs/composefs-rs", rev = "b928c6bd6c051e111d3efc3d25cdaf9159182ed0", package = "composefs", features = ["rhel9"] }
 cfsctl = { git = "https://github.com/composefs/composefs-rs", rev = "b928c6bd6c051e111d3efc3d25cdaf9159182ed0", package = "cfsctl", features = ["rhel9"] }

crates/lib/src/bootc_composefs/boot.rs

@@ -730,7 +730,7 @@ pub(crate) fn setup_composefs_bls_boot(
     let (config_path, booted_bls) = if is_upgrade {
         let boot_dir = Dir::open_ambient_dir(&entry_paths.config_path, ambient_authority())?;
 
-        let mut booted_bls = get_booted_bls(&boot_dir)?;
+        let mut booted_bls: BLSConfig = get_booted_bls(&boot_dir)?;
         booted_bls.sort_key = Some(secondary_sort_key(&os_id));
 
         let staged_path = loader_path.join(STAGED_BOOT_LOADER_ENTRIES);