chore(ci): consolidate docker dependabot entries using directories

[pront]

Summary

Consolidate the four separate package-ecosystem: docker entries in .github/dependabot.yml into a single entry using the directories key. This reduces 41 lines of duplication while preserving identical behavior — same schedule, labels, commit-message prefix, PR limit, and grouping for all four Docker image directories.

Vector configuration

N/A

How did you test this PR?

Reviewed the Dependabot directories docs to confirm the key is supported for package-ecosystem: docker. No functional change — dependabot will continue scanning the same four paths.

Change Type

• Bug fix
• New feature
• Dependencies
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

[datadog-vectordotdev]

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