Release v3.0.4

• Automatically build Dockerfile.samus as tempfile if it is not present in
  the repo when using Samus::Rake::DockerReleaseTask. This docker image
  copies all credentials in so it can be run directly without mounts.
• Add mount_samus_config option (defaults to false) to DockerReleaseTask
  options to allow Docker image to mount the Samus configuration directory
  from the host when publishing the image. To override the config directory,
  specify the SAMUS_CONFIG_PATH environment variable to the publish task.
• Add extra_config to DockerReleaseTask to allow extra files to be
  copied into the /root directory of the build image. The value should be
  a hash of src -> dest filenames to copy.