Fix DockerComposeContainer to support compose files from different directories

[snehal242005]

Closes #1863

What does this PR do?

DockerComposeContainer (containerized mode) previously determined the working
directory / root for docker-compose using only the first compose file's parent
directory. When additional compose files lived in a different directory, that
directory was never copied into the container, causing a FileNotFoundError.

This PR:

• Adds PathUtils.findCommonParent(List<File>), which computes the closest
  common ancestor directory of all provided compose files.
• Updates ContainerisedDockerCompose to use this common parent as the root
  directory that gets copied into the container, instead of just the first
  file's directory — ensuring every provided compose file is reachable.
• Fixes a related bug where, when withCopyFilesInContainer was used, only
  the base compose file was copied into the container individually — other
  compose files were silently dropped even in the same-directory case. All
  compose files are now copied correctly.
• Behavior is unchanged when all compose files share one directory (the
  common case) — findCommonParent reduces to that same directory.

How to test?

1. Create two docker-compose files in different directories, e.g.:
   • first/directory/docker-compose.yml
   • first/docker-compose.override.yml
2. Construct a DockerComposeContainer with both files
3. Previously: FileNotFoundError. Now: both files resolve and the container
   starts correctly.

Tests

Added PathUtilsTest.java covering:

• Single file → its own directory
• Files in the same directory → that directory
• The issue's exact scenario (files in different directories) → correct
  common parent
• Order independence
• Sibling directories → shared parent

All 5 tests pass (./gradlew :testcontainers:test --tests PathUtilsTest).

Mandatory checks

• Tests created for changes
• Manually verified logic against the exact scenario from the issue
• No changes needed to documentation