feat: add build-push secret passthrough inputs

[lifeiscontent]

Summary

• add BuildKit secret passthrough inputs to the build-push action: secrets, secret-envs, and secret-files
• update the action internals to newer dependency versions and fix branch tag detection by setting TARGET_BRANCH
• refresh the README examples to cover the new inputs and unreleased default-branch usage

Consumer PR

• plane-ee integration example: https://github.com/makeplane/plane-ee/pull/6539

Validation

• ruby -e "require "yaml"; YAML.load_file("build-push/action.yml"); puts "ok""
• git diff --check