Add `--ignore-file` flag for .dockerignore, .prettierignore, etc.

[kehoecj]

Description

The --gitignore flag lets users skip files matched by .gitignore patterns. Many projects also have other ignore files (.dockerignore, .prettierignore, .eslintignore) that use the same gitignore syntax. A new --ignore-file flag would let users apply these patterns during file discovery.

Proposed behavior

# Skip files matched by .dockerignore
validator --ignore-file=.dockerignore .

# Multiple ignore files (patterns are additive)
validator --ignore-file=.dockerignore --ignore-file=.prettierignore .

• Repeatable flag, each file adds more patterns
• Files use gitignore syntax (same parser as --gitignore)
• Paths resolved relative to the search path root
• Missing files silently skipped (lenient)
• Config file support: ignore-files = [".dockerignore"] in .cfv.toml
• Env var: CFV_IGNORE_FILES (comma-separated)