[Duo-ee]

Description

The page opens with an [!IMPORTANT] block stating that "all paths must be
defined relative to the base file", which contradicts the ## Relative paths
section further down the same page. #26501
reports the contradiction and asks which one is right.

The ## Relative paths section is right. The file path of an extends
attribute is resolved relative to the main Compose file, while relative paths
declared by the service being extended are resolved relative to the Compose file
that declares them. The example on that page already shows the outcome
(env_file: ./container.env inside commons/compose.yaml becomes
../commons/container.env).

This change keeps the section, and replaces the incorrect warning with a note
that states the actual behavior. Deleting the warning outright was the first
option in the issue, but the note also carries the part that is true: file
itself is relative to the main Compose file.

Evidence from the Compose specification and compose-go v2.16.2

• The Compose specification, 05-services.md under "Finding referenced
  service": a relative file path "is considered as relative to the location of
  the main Compose file".
• compose-go v2.16.2, the version pinned by docker/compose's go.mod:
  • loader/extends.go loads the referenced file with ResolvePaths = false
    and a comment saying "we do relative path resolution after file has been
    loaded", then calls paths.ResolveRelativePaths(source, relworkingdir, remotes).
  • relworkingdir is loader.Dir(refPath), the directory of the referenced
    file, not the directory of the main Compose file.
  • paths/resolve.go joins each relative path (services.*.env_file.*.path,
    services.*.volumes.*, services.*.build.context, and so on) onto that
    directory.
  • services.*.extends.file has its own resolver, so a nested extends.file
    is resolved against the file that declares it.
• Merging several files with -f is the case where paths resolve against the
  first file, which merge.md documents. The warning here applied that rule to
  extends.

Verification

• npx --no-install rumdl fmt and npx --no-install rumdl check on the
  changed file: no issues.
• vale with the repository configuration on the changed file: 0 errors, 0
  warnings and 0 suggestions. The i.e. in the old wording is gone with the
  rewrite.
• Docker isn't available in this environment, so docker buildx bake validate
  wasn't run locally.

Related issues or tickets

Closes #26501

Reviews

• Technical review
• Editorial review
• Product review

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