Release v3.0.3

• Add Samus::Rake::ReleaseTask and Samus::Rake::DockerReleaseTask to
  generate helpful Rake tasks to generate releases. Example:

require 'samus'

Samus::Rake::ReleaseTask.new do |t|
  t.git_pull_after_release = true # default is true
  t.zipfile = "customzip.tar.gz"  # default release-vX.Y.Z.tar.gz
  t.buildfile = "samus.json"      # default is samus.json
end

• Add lsegal/samus:build Dockerfile to simplify creation of build docker images.