Support CNB extensions when building images

[scottfrederick]

Add support for Cloud Native Buildpack extensions when building images.

Depends on #41352

[wilkinsona]

See https://github.com/spring-projects/spring-boot/issues/42590#issue-2579203360 for an example of why this would be useful.

[pr11me]

Here's another use case: building the image in CI without internet access, e.g. due to corporate proxy or air-gapped scenario.

The problem here is that the builder tries to download a JRE/JDK from somewhere during the build process.

Using extensions, one could extend the run image with a JRE and avoid the download.

[LightGuard]

Has there been an update to this?

[philwebb]

@LightGuard No update I'm afraid, it's still on the TODO list.