feat(cli): support Containerfile and custom files in sandbox create

[tirthpatel90]

Fixes #2420

Summary

Updates the --from path resolution logic in openshell-cli to treat any existing valid file path as a container build file, removing rigid Dockerfile name restrictions.

Changes

• Updated file resolution in resolve_from to accept any existing file passed to --from without requiring a Dockerfile name convention.
• Updated directory resolution to fall back to Containerfile if Dockerfile is absent.
• Updated filename_looks_like_dockerfile to include containerfile and .containerfile.
• Updated error messages to include Containerfile.

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[tirthpatel90]

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