Fix Docker multi-architecture image platform handling in buildpack operations

[hojooo]

Summary

This PR fixes Docker platform handling issues that occur when building images for a platform different from the host platform. The changes ensure that platform information is properly propagated throughout the buildpack layer export and multi-architecture image inspection processes.

Related Issues

• New arm64 macbooks fail to bootBuildImage due to incorrect platform image #46665: New arm64 macbooks fail to bootBuildImage due to incorrect platform image
• Image building may fail when specifying a platform if an image has already been built with a different platform #46674: Image building may fail when specifying a platform if an image has already been built with a different platform

Issue Description

Two related issues were affecting users building container images with Spring Boot's buildpack integration:

1. New arm64 macbooks fail to bootBuildImage due to incorrect platform image #46665: ARM64 Mac users failed to build AMD64 platform images due to incorrect platform handling in Docker API calls. When buildpacks needed to be downloaded from Docker Hub, the docker save command didn't include platform information, causing Docker daemon to default to the host platform (ARM64) instead of the requested platform (AMD64).
2. Image building may fail when specifying a platform if an image has already been built with a different platform #46674: Image platform mismatch errors occurred when building images with different platforms consecutively. Users experienced failures with the message "Image platform mismatch detected" when switching between platforms without clearing Docker's cache.

Root Cause

The issues stemmed from incomplete platform information propagation in the Docker API implementation:

• During buildpack layer export, platform information wasn't passed to the docker save command
• Multi-architecture image inspection didn't properly respect the pulled digest, leading to platform conflicts

Changes Made

1. Respect pulled digest when inspecting multi-arch images

• File: buildpack/spring-boot-buildpack-platform/src/main/java/org/springframework/boot/buildpack/platform/build/Builder.java
• Change: Modified image inspection logic to use the actual pulled digest instead of the manifest tag
• Impact: Prevents platform mismatch errors when working with multi-architecture images

2. Respect image platform when exporting buildpack layers

• File: buildpack/spring-boot-buildpack-platform/src/main/java/org/springframework/boot/buildpack/platform/docker/DockerApi.java
• Change: Enhanced Docker save operations to include platform information
• Impact: Ensures correct platform handling during buildpack layer export

[hojooo]

#46674 (comment)