Fix explicit compose file path resolution

[ravshansbox]

Summary

• honor explicit -f/--file input instead of treating compose.yml as a sentinel
• autodiscover standard compose filenames only when no file option was provided
• resolve compose-relative env_file, build context, and Dockerfile paths from the compose file directory

Problem

The existing code always derived composePath from "\(cwd)/\(composeFilename)", which breaks absolute compose file paths and conflates default behavior with explicit user input.

Changes

• change composeFilename from a magic default string to String? in ComposeUp and ComposeDown
• add a shared resolvedPath(for:relativeTo:) helper
• move compose autodiscovery into composePath and run it only when no explicit file was passed
• resolve service env_file, build context, and Dockerfile paths relative to the compose file directory in ComposeUp

Verification

• swift build
• ./.build/debug/Container-Compose down -f /Users/user/projects/project/docker-compose.yml

Fixes #63

[Mcrich23]

The original intent of this flag was for it to be relative for those who want a different compose file in their folder. I totally see why this could be frustrating for someone who may want to use an absolute path instead.

Let me do some code review and we can move forward from there.

[Mcrich23]

It looks pretty good, just a couple issues. Thanks for working on this!