authorized_keys.rb

action :append do
  home = "/home/#{new_resource.user}"

  directory "#{home}/.ssh" do
    owner new_resource.user
    group new_resource.group
    mode 0700
    action :create
    only_if "test -d #{home}"
  end

  f = file "#{home}/.ssh/authorized_keys" do
    owner user
    group group
    content new_resource.public_keys.join("\n")
  end

  new_resource.updated_by_last_action(f.updated_by_last_action?)
end

resource.rb

actions :append
default_action :append

attribute :user, kind_of: [String, NilClass]
attribute :group, kind_of: [String, NilClass]
attribute :mode, kind_of: [Integer, NilClass]
attribute :ssh_keys, kind_of: Array