docs: function getLayersFromManifest(manifest) {
      var i = 0;
      if (manifest.schemaVersion === 1) {
          return manifest.fsLayers.map(function (layer) {
              i += 1;
              return {
                  i: i,
                  digest: layer.blobSum
              };
          });
      }
      assert.equal(manifest.schemaVersion, 2, 'manifest.schemaVersion === 2');
      return manifest.layers.map(function (layer) {
          i += 1;
          return {
              i: i,
              digest: layer.digest
          };
      });
  }