get_latest.rb

#!/usr/bin/env ruby

require 'uri'
require 'net/http'
require 'json'

DOCKER_SRC = %r{ghcr\.io/([\w-]+)/([\w-]+):([\w-]+)}

def parse_source(src)
  match = src.match(DOCKER_SRC)
  raise('failed to parse package') unless match
  match.captures
end

def make_request(org, image)
  uri = URI("https://api.github.com/orgs/#{org}/packages/container/#{image}/versions")
  req = Net::HTTP::Get.new(uri)
  req['Accept'] = 'application/vnd.github.v3+json'
  req['Authorization'] = "token #{ENV['GITHUB_TOKEN']}"

  resp = Net::HTTP.start(uri.hostname, uri.port, use_ssl: true) do |http|
    http.request(req)
  end
  return JSON.parse(resp.body) if resp.code_type == Net::HTTPOK
  raise("Failed API lookup: #{resp.code}")
end

def get_latest(src)
  org, image, = parse_source(src)
  resp = make_request(org, image)
  tags = resp.first.dig('metadata', 'container', 'tags')
  # this checks if latest is in the tag list *and* removes it, leaving just the datestamp tag
  raise("Latest image doesn't have latest tag") unless tags.delete 'latest'
  raise('No valid tag found') if tags.first.nil?
  return tags.first
end

if caller.empty?
  src = ARGV.shift || raise('image path not provided')
  puts get_latest(src)
end