code: func (b *Builder) Step() *Step {
  	dst := make([]string, len(b.Env)+len(b.RunConfig.Env))
  	copy(dst, b.Env)
  	dst = append(dst, b.RunConfig.Env...)
  	dst = append(dst, b.Arguments()...)
  	return &Step{Env: dst}
  }
docs: // Step creates a new step from the current state.