feat(ecr-assets): add support for docker build context

[cogwirrel]

Issue # (if applicable)

Fixes #31598

Reason for this change

Add support for docker's --build-context flag in docker builds. This is useful for a few reasons outlined in the linked issue (and other similar issues), such as:

• Sharing files from directories outside the Docker build directory
• Using specific image versions as build contexts (docker-image://alpine:latest)
• Referencing remote URLs as build contexts

Description of changes

Adds support for Docker's --build-context flag when building Docker image assets. This allows users to specify additional named build contexts that can be referenced in Dockerfiles via COPY --from=<name>.

• Added buildContexts (optional Record<string, string>) to DockerBuildOptions, DockerImageAssetOptions, DockerImageAssetInvalidationOptions, and DockerImageAssetSource
• Updated DockerImage.fromBuild() to pass --build-context key=value flags to the docker build command
• Wired buildContexts through the full asset pipeline: DockerImageAsset → synthesizer → asset manifest → cloud assembly schema
• Added token validation for buildContexts keys and values (same as buildArgs)
• Added buildContexts to asset hash invalidation (controllable via invalidation.buildContexts)
• Added ASSET_RESOURCE_METADATA_DOCKER_BUILD_CONTEXTS_KEY metadata constant
• Updated the aws-ecr-assets README with documentation and usage example

In terms of design decisions, this follows the same pattern as buildArgs.

This requires the CLI changes in aws/aws-cdk-cli#1128 to be released first.

Describe any new or updated permissions being added

N/A

Description of how you validated changes

Integration test, using linked local AWS CDK CLI with changes from aws/aws-cdk-cli#1128

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[aws-cdk-automation]

(This review is outdated)

[github-actions]

⚠️ Experimental Feature: This security report is currently in experimental phase. Results may include false positives and the rules are being actively refined.
Please try merge from main to avoid findings unrelated to the PR.

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	Tests	Passed ✅	Skipped	Failed
Security Guardian Results	48 ran	48 passed

Test	Result
No test annotations available

• View Security Guardian Results

[github-actions]

⚠️ Experimental Feature: This security report is currently in experimental phase. Results may include false positives and the rules are being actively refined.
Please try merge from main to avoid findings unrelated to the PR.

---

	Tests	Passed ✅	Skipped	Failed
Security Guardian Results with resolved templates	48 ran	48 passed

Test	Result
No test annotations available

• View Security Guardian Results with resolved templates

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.