fix: resolve gemspec load path and refactor proxy config method

[damacus]

• Change File.expand_path to use dir instead of FILE in gemspec to properly resolve the lib directory path
• Extract repetitive proxy environment variable logic into separate proxy_env_vars helper method
• Reduce dockerfile_proxy_config ABC size from 24.21 to below threshold
• Reduce method length from 14 lines to 4 lines to meet style guidelines

Signed-off-by: Dan Webb [email protected]

[damacus]

@copilot look at how we do things in https://github.com/test-kitchen/kitchen-vagrant/

fix this chefstyle issue

[Copilot]

@damacus I've opened a new pull request, #441, to work on those changes. Once the pull request is ready, I'll request review from you.