fix: Do not pull base image with unresolved platform variable

[HofmeisterAn]

What does this PR do?

The PR fixes the base image pull for Dockerfiles that set FROM --platform with a built-in build argument such as $BUILDPLATFORM or $TARGETPLATFORM.

Built-in build arguments are not declared with ARG, so the variable was sent to the Docker daemon as is, and the pull failed with "$BUILDPLATFORM" is an invalid OS component before the build started. This broke the common cross-compilation pattern FROM --platform=$BUILDPLATFORM ... AS build whenever the base image was not already in the image store.

DockerfileArchive.GetBaseImages now takes the target platform of a single-platform build. A base image without a --platform flag uses the target platform, $TARGETPLATFORM resolves to it, and a variable that does not resolve, such as $BUILDPLATFORM, falls back to the platform of the Docker host.

Why is it important?

-

Related issues

• Relates feat: Add Platform property to IImage interface #1610

Summary by CodeRabbit

• Bug Fixes
  • Improved base-image platform detection during image builds. Images without an explicitly declared platform now use the selected target platform, including when the Dockerfile references TARGETPLATFORM.
  • Preserved explicit platform settings and build-platform behavior when identifying base images. This helps ensure the correct base images are selected for builds that use platform variables or multiple build platforms.

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📥 Commits

Reviewing files that changed from the base of the PR and between 5d4d0f5 and 713c876.

📒 Files selected for processing (2)

• src/Testcontainers/Images/DockerfileArchive.cs
• tests/Testcontainers.Tests/Assets/pullBaseImages/Dockerfile

🚧 Files skipped from review as they are similar to previous changes (2)

• tests/Testcontainers.Tests/Assets/pullBaseImages/Dockerfile
• src/Testcontainers/Images/DockerfileArchive.cs

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Walkthrough

Base-image discovery now accepts a target platform and uses it to resolve base-image platforms. Build preparation passes the target platform for single-platform builds. Tests cover platform selection for Dockerfile stages.

Changes

Base-image platform resolution

Layer / File(s)	Summary
Resolve base-image platforms src/Testcontainers/Images/DockerfileArchive.cs, src/Testcontainers/Images/Platform.cs, tests/Testcontainers.Tests/Assets/pullBaseImages/Dockerfile, tests/Testcontainers.Tests/Unit/Images/ImageFromDockerfileTest.cs	GetBaseImages accepts an optional target platform. It adds TARGETPLATFORM to the argument values, with Dockerfile ARG values and then build arguments taking precedence. Only ARG instructions before the first FROM are used for FROM substitution. Images without a declared platform use the target platform. Declared platform values containing $ resolve to no platform. The tests cover platform arguments, explicit platform values, and the added Alpine SDK and runtime images.
Pass the build platform src/Testcontainers/Clients/TestcontainersClient.cs	PrepareBuildAsync passes the single build platform to GetBaseImages. It passes null when the platform is empty or comma-separated.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~15 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to 713c8

Base-image platform selection appears consistent with the intended build behavior. No issue requiring a change before merge was identified.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 5d4d0

The change stays within the existing Docker build and registry-authentication boundaries. No introduced security vulnerability was established, but platform selection still depends on shared image-cache behavior and agreement between the Docker host and builder.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The directly affected assets are base images in the configured Docker daemon's shared image store. Builds using unresolved platform variables can now successfully pre-pull images using existing registry credentials; no broader environment or cross-tenant exposure was established.

Trust Boundaries and Controls

• observed — Image pulls continue to select authentication by the image's registry hostname, with the daemon's default registry used for unqualified names. The selected credentials and discovered platform are passed through the existing Docker image-creation API.

Resilience and Maintainability Implications

• observed — Pre-pull cache checks use image names without platform, and concurrent pulls have no compensating cleanup for successful pulls when another fails or cancellation occurs. These mechanisms predate the PR. Preparation failure prevents the subsequent build, but already-pulled images can remain; precise concurrent daemon outcomes were not verified.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 4 files. (1 skipped: 1…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: preventing base-image pulls from using an unresolved platform variable.
Description check	✅ Passed	The description explains the change and its motivation, and it references related issue #1610. The “Why is it important?” section contains only a dash, but the rationale is explained in the “What does…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 4 files. (1 skipped: 1 unsupported.)

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
Review comments at @src/Testcontainers/Images/DockerfileArchive.cs:
- Around line 169-170: Limit the `FROM` argument defaults used by the
`builtInArgs` and `parsedArguments` aggregation to `ARG` declarations before the
first `FROM`; exclude stage-local declarations so they cannot override automatic
platform arguments used to resolve the base image.

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ℹ️ Review info

⚙️ Run configuration

• Configuration used: Repository UI
• Review profile: CHILL
• Plan: Advanced
• Run ID: 10a7e07a-e4b0-488a-8569-27407e59bf85

📥 Commits

Reviewing files that changed from the base of the PR and between 053ffb9 and 5d4d0f5.

📒 Files selected for processing (5)

• src/Testcontainers/Clients/TestcontainersClient.cs
• src/Testcontainers/Images/DockerfileArchive.cs
• src/Testcontainers/Images/Platform.cs
• tests/Testcontainers.Tests/Assets/pullBaseImages/Dockerfile
• tests/Testcontainers.Tests/Unit/Images/ImageFromDockerfileTest.cs

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