fix: use path.Join instead of url.JoinPath when prepending a custom registry to an image

[fedorkanin]

What does this PR do?

This MR swaps url.JoinPath with path.Join in image substitutors.

Why is it important?

url.JoinPath uses url.Parse internally, which doesn't correctly parse URLs like example.com:443 without a schema. Docker image reference cannot have a schema [docs]. Incorrectly parsed URL inside url.JoinPath causes the image tag (second argument) to be completely ignored [go playground], which results in 404 errors when pulling an image.

Related issues

• Closes [Bug]: Incorrect TESTCONTAINERS_HUB_IMAGE_NAME_PREFIX parsing causes 404 on image pull #3306.
• Relates [Bug]: TESTCONTAINERS_HUB_IMAGE_NAME_PREFIX trailing slash not handled properly #2746.

How to test this PR

I've added a test case to image_substitutors_test.go:

go test -v -run TestPrependHubRegistrySubstitutor

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[coderabbitai]

Summary by CodeRabbit

• Bug Fixes

  • Correctly handle container registries that include a port when prepending a hub/registry to image names, preventing malformed image references.

• Improvements

  • More consistent and reliable joining of registry and image paths, reducing edge-case errors.

• Tests

  • Added test coverage for registry-with-port scenarios to ensure accurate image substitution.

Walkthrough

Replaced use of net/url's JoinPath with path.Join for image substitutors, removed related error handling and net/url import, and added a test verifying substitution when the registry prefix includes a port (e.g., "my-registry:5000/foo:latest").

Changes

Cohort / File(s)	Summary
Image substitutor implementation options.go	Replaced url.JoinPath with path.Join in CustomHubSubstitutor.Substitute and prependHubRegistry.Substitute; removed net/url import and error handling; added path import; functions now return joined path and nil error.
Tests image_substitutors_test.go	Added subtest "registry-with-port" to TestPrependHubRegistrySubstitutor validating that newPrependHubRegistry("my-registry:5000") substitutes "foo:latest" to "my-registry:5000/foo:latest".

Sequence Diagram(s)

sequenceDiagram
  autonumber
  participant Caller
  participant Substitutor
  Note right of Substitutor: Old flow (url.JoinPath)\ncould drop tag when prefix has port
  Caller->>Substitutor: Substitute(prefix, image)
  alt old (url.JoinPath)
    Substitutor-->>Substitutor: url.Parse(prefix)  (may treat as opaque -> drop second arg)
    Substitutor-->>Caller: joinedPathOrEmpty + error handling
  else new (path.Join)
    Substitutor-->>Substitutor: path.Join(prefix, image)
    Substitutor-->>Caller: "prefix/image" (nil error)
  end
  Note left of Caller: Test validates "my-registry:5000/foo:latest"

Loading

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

I nudge the tags with gentle hop,
No scheme, just paths—clean chop-chop!
Ports now nest where images roam,
Join the path and bring them home.
Tests nibble bugs that used to hide—🐇✨

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title Check	✅ Passed	The title succinctly and accurately describes the primary change of replacing url.JoinPath with path.Join when prepending a custom registry to an image, reflecting the core update in the pull request without extraneous detail.
Linked Issues Check	✅ Passed	The pull request implements the coding objectives from issue #3306 by replacing url.JoinPath with path.Join, adding a targeted test to cover registries with ports, and closing the issue, thereby addressing the regression and preserving image tags as required.
Out of Scope Changes Check	✅ Passed	All modified files and added tests pertain exclusively to the registry path-joining logic for image substitutors, and there are no edits outside the scope of fixing url.JoinPath usage and validating the correction.
Description Check	✅ Passed	The description directly explains the swap from url.JoinPath to path.Join, the rationale based on Docker image reference parsing issues, and includes testing instructions, which all relate to the actual code changes in the pull request.

✨ Finishing touches

• 📝 Generate Docstrings

🧪 Generate unit tests

• Create PR with unit tests
• Post copyable unit tests in a comment

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📜 Recent review details

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📥 Commits

Reviewing files that changed from the base of the PR and between d4fc16c and 193b35f.

📒 Files selected for processing (1)

• image_substitutors_test.go (1 hunks)

🚧 Files skipped from review as they are similar to previous changes (1)

• image_substitutors_test.go

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[mdelapenya]

LGTM, thanks for the detailed report, and for contributing a fix. 🙇

[mdelapenya]

Merged, thank you so much for your work here 🙇