image-oci: add multi-layer OCI image support with OCI_LAYERS

Add support for creating multi-layer OCI images with explicit layer
definitions via OCI_LAYERS variable. This enables fine-grained control
over container layer composition.

New variables:
- OCI_LAYER_MODE: Set to "multi" for explicit layer definitions
- OCI_LAYERS: Define layers as "name:type:content" entries
  - packages: Install specific packages in a layer
  - directories: Copy directories from IMAGE_ROOTFS
  - files: Copy specific files from IMAGE_ROOTFS

Package installation uses Yocto's package manager classes (RpmPM,
OpkgPM) for consistency with do_rootfs, rather than calling dnf/opkg
directly.

Example usage:
  OCI_LAYER_MODE = "multi"
  OCI_LAYERS = "\
      base:packages:base-files+base-passwd+netbase \
      shell:packages:busybox \
      app:packages:curl \
  "

This creates a 3-layer OCI image with discrete base, shell, and app
layers that can be shared and cached independently.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>

1 files changed, 38 insertions, 0 deletions

diff --git a/recipes-demo/images/app-container-multilayer.bb b/recipes-demo/images/app-container-multilayer.bb
new file mode 100644
index 00000000..6a3f2042
--- /dev/null
+++ b/recipes-demo/images/app-container-multilayer.bb
@@ -0,0 +1,38 @@
+SUMMARY = "Multi-layer Application container - test OCI_LAYERS"
+DESCRIPTION = "Demonstrates OCI_LAYER_MODE = 'multi' with explicit layer definitions"
+LICENSE = "MIT"
+LIC_FILES_CHKSUM = "file://${COREBASE}/meta/COPYING.MIT;md5=3da9cfbcb788c80a0384361b4de20420"
+
+# Multi-layer mode: create explicit layers instead of single rootfs layer
+OCI_LAYER_MODE = "multi"
+
+# Define layers: each layer contains specific packages
+# Format: "name:type:content" where content uses + as delimiter for multiple items
+OCI_LAYERS = "\
+    base:packages:base-files+base-passwd+netbase \
+    shell:packages:busybox \
+    app:packages:curl \
+"
+
+# Use CMD so `docker run image /bin/sh` works as expected
+OCI_IMAGE_CMD = "/bin/sh -c 'echo Hello from multi-layer container && curl --version'"
+
+IMAGE_FSTYPES = "container oci"
+inherit image
+inherit image-oci
+
+IMAGE_FEATURES = ""
+IMAGE_LINGUAS = ""
+NO_RECOMMENDATIONS = "1"
+
+# IMAGE_INSTALL triggers package builds via do_rootfs recrdeptask.
+# Even for multi-layer mode, list packages here to ensure they're built.
+# The PM will install them directly to layers from DEPLOY_DIR_IPK.
+# Note: IMAGE_ROOTFS is still created but ignored for packages layers.
+IMAGE_INSTALL = "base-files base-passwd netbase busybox curl"
+
+# Allow build with or without a specific kernel
+IMAGE_CONTAINER_NO_DUMMY = "1"
+
+# Note: No ROOTFS_POSTPROCESS_COMMAND needed - IMAGE_ROOTFS is empty
+# and PM handles installation directly to OCI layers