Platform API changes for image extension build contexts

Co-authored-by: Pavel Busko <pavel.busko@sap.com>

image_extension.md

@@ -89,9 +89,27 @@ Correspondingly, each `/bin/generate` executable:
 - MAY log output from the build process to `stdout`.
 - MAY emit error, warning, or debug messages to `stderr`.
 - MAY write either or both of `build.Dockerfile` and `run.Dockerfile` to the `<output>` directory. This file MUST adhere to the requirements listed below.
+- MAY create the following folders in the `<output>` directory with an arbitrary content:
+
+  either:
+
+  - `context`
+
+  or the image-specific folders:
+
+  - `context.run`
+  - `context.build`
 - MAY write key-value pairs to `<output>/extend-config.toml` that are provided as build args to build.Dockerfile when extending the build image.
 - MUST NOT write SBOM (Software-Bill-of-Materials) files as described in the [Software-Bill-of-Materials](#software-bill-of-materials) section.
 
+#### Context Folders
+
+- The `<output>/context` folder MUST NOT be created together with any combination of the image-specific folders.
+- If the folder `<output>/context` is present it will be set as Kaniko build context during the `extend` phase of the build and run images.
+- If the folder `<output>/context.run` is present it will be set as Kaniko build context during the `extend` phase of the run image only.
+- If the folder `<output>/context.build` is present it will be set as Kaniko build context during the `extend` phase of the build image only.
+- If none of these folders is not present, Kaniko build context defaults to the `<app>` folder.
+
 #### Dockerfile Requirements
 
 A `run.Dockerfile`

platform.md

@@ -605,14 +605,15 @@ When extending the build image:
 | `1-10`, `13-19` | Generic lifecycle errors            |
 | `100-109`       | Extension-specific lifecycle errors |
 
-- For each extension in `<group>` in order, if a Dockerfile exists in `<generated>/<kind>/<buildpack-id>`, the lifecycle:
+- For each extension in `<group>` in order, if a Dockerfile exists in `<generated>/<buildpack-id>/<kind>.Dockerfile`, the lifecycle:
   - SHALL apply the Dockerfile to the environment according to the process outlined in the [Image Extension Specification](image-extension.md).
+  - SHALL set the Kaniko build context to the folder according to the process outlined in the [Image Extension Specification](image-extension.md).
 - The extended image MUST be an extension of:
   - The `build-image` in `<analyzed>` when `<kind>` is `build`, or
   - The `run-image` in `<analyzed>` when `<kind>` is `run`
-- When extending the build image, after all `build.Dockefile`s are applied, the lifecycle:
+- When extending the build image, after all `build.Dockerfile`s are applied, the lifecycle:
   - SHALL proceed with the `build` phase using the provided `<gid>` and `<uid>`
-- When extending the run image, after all `run.Dockefile`s are applied, the lifecycle:
+- When extending the run image, after all `run.Dockerfile`s are applied, the lifecycle:
   - **If** any `run.Dockerfile` set the label `io.buildpacks.rebasable` to `false` or left the label unset:
     - SHALL set the label `io.buildpacks.rebasable` to `false` on the extended run image
   - **If** after the final `run.Dockerfile` the run image user is `root`,