fix(docker_compose): support folded scalar and docker.io-prefixed image values

[thavaahariharangit]

What are you trying to accomplish?

Resolves #13351

Adds support in the docker_compose file updater for image values written as folded/literal YAML block scalars (e.g. image: >- on its own line) and for values explicitly prefixed with docker.io/. Previously these forms were not matched by the updater’s replacement regex, causing the update to fail with Expected content to change! and preventing a PR from being created.

Anything you want to highlight for special attention from reviewers?

How will you know you've accomplished your goal?

Reproduced the failure in a minimal workflow: https://github.com/thavaahariharangit/docker-compose-core-issue-13351/actions/runs/26227433049/job/77178148320

Before the fix, the updater errored out and no PR was created:

Dependabot encountered '1' error(s) during execution, please check the logs for more details.
+-----------------------------------------------+
|         Dependencies failed to update         |
+---------------+---------------+---------------+
| Dependency    | Error Type    | Error Details |
+---------------+---------------+---------------+
| library/nginx | unknown_error | null          |
+---------------+---------------+---------------+

After the fix, running the updater locally against the same repo produces the expected PR:

updater | 2026/05/21 13:03:11 INFO Results:
updater | +---------------------------------------------------+
updater | |        Changes to Dependabot Pull Requests        |
updater | +---------+-----------------------------------------+
updater | | created | library/nginx ( from 1.25.4 to 1.31.0 ) |
updater | +---------+-----------------------------------------+

A regression spec covering the folded scalar image: form with a docker.io/library/nginx:<tag>@sha256:<digest> value has been added, and the full docker and docker_compose file updater specs pass.

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.

[Copilot]

Pull request overview

This PR fixes Docker Compose image updates that previously failed when the image: value was expressed as a YAML folded/literal block scalar (e.g., image: >- on its own line) and when image references were explicitly prefixed with docker.io/. It updates the docker-compose updater logic and its shared YAML image replacement regex so Dependabot can correctly rewrite these forms instead of raising Expected content to change!.

Changes:

• Extend the shared YAML image replacement logic to handle docker.io/-prefixed images and block scalar image: values.
• Add a docker-compose updater fallback path: try the existing “dockerfile-style” replacement first, then fall back to YAML-image updating when no changes were made.
• Add a regression fixture and spec covering a folded scalar image: with a docker.io/library/nginx:<tag>@sha256:<digest> value.

Show a summary per file

File	Description
docker/lib/dependabot/shared/shared_file_updater.rb	Enhances YAML image: replacement to preserve docker.io/ prefix and support folded/literal block scalar forms.
docker/lib/dependabot/docker_compose/file_updater.rb	Adds a fallback strategy to update compose files via YAML logic when the existing replacement path makes no changes.
docker/spec/dependabot/docker_compose/file_updater_spec.rb	Adds a regression spec for folded-scalar image: values (with docker.io/ prefix + digest).
docker/spec/fixtures/docker_compose/composefiles/folded_digest	New fixture representing a folded-scalar compose image: value with docker.io/ prefix and digest.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 2

[Copilot]

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 2