Mapping ratio function over ImageCollection in Google Earth Engine

As you calculated pixel values in each point and put them in B8_list, you don't need to use again 'ee.Reducer' in your function. You only need to map Image Collection as follows. I assumed an arbitrary point (pt) and polygon in Spain and an arbitrary range for start and finish dates.

var pt = ee.Geometry.Point([-1.869490554037725, 41.191353356841205]);

var polygon = ee.Geometry.Polygon(
        [[[-1.8966130515963187, 41.18786546760974],
          [-1.8736104270846, 41.173136775751885],
          [-1.8413380882174124, 41.176754656054],
          [-1.8389348289400687, 41.19974936510326],
          [-1.861250807943975, 41.207498570723196],
          [-1.8732671043306937, 41.19690775979286],
          [-1.8883733055025687, 41.2000076867413]]]);

Map.addLayer(pt);

var start = '2017-01-01';
var finish = '2017-06-01';

var l8 = ee.ImageCollection('LANDSAT/LC08/C01/T1_TOA')
  .filterDate (start,finish)
  .filterBounds(pt)
  .filter("WRS_ROW <= 122");
  //.map(maskL8);

var myB8 = l8.select("B8");

//receiving the values at pt for each image

var getB8 = function(image) {

  // Reducing region and getting value
  var value_B8 = ee.Image(image)
    .reduceRegion(ee.Reducer.first(), pt)
    .get('B8');

  return value_B8;
};

var count = myB8.size();

var listOfImages = l8.toList(count);

var B8_list = listOfImages.map(getB8);

print("B8 list", B8_list);

//Ratio function

function  calculateRatio (image) {
  
  var time = ee.Image(image).get('system:time_start');
  
  var idx = ee.List(listOfImages).indexOf(image);
  
  var ratio1 = ee.Image(image).select('B8')
                              .clip(polygon)
                              .divide(ee.Number(B8_list.get(idx)));
    
    return ratio1.set('system:time_start', time);

}

print("Original Image Collection", myB8);

var RatioALL = listOfImages.map(calculateRatio);

print ("Ratio All", RatioALL);

After running above code in GEE code editor, I got following result.

It can be observed that each obtained image was divided by its respective point pixel value and it also has a 'system:time_start'.

Editing Note:

For avoiding nulls in B8_list you can try following code. I used my initial parameters because when I implemented mask function in my code null values also appear.

var maskL8 = function(image) {
  var qa = image.select('BQA');

  var mask = qa.bitwiseAnd(1 << 4).eq(0);
  return image.updateMask(mask);
};

var pt = ee.Geometry.Point([-1.869490554037725, 41.191353356841205]);

var polygon = ee.Geometry.Polygon(
        [[[-1.8966130515963187, 41.18786546760974],
          [-1.8736104270846, 41.173136775751885],
          [-1.8413380882174124, 41.176754656054],
          [-1.8389348289400687, 41.19974936510326],
          [-1.861250807943975, 41.207498570723196],
          [-1.8732671043306937, 41.19690775979286],
          [-1.8883733055025687, 41.2000076867413]]]);

Map.addLayer(pt);

var start = '2017-01-01';
var finish = '2017-06-01';

var l8 = ee.ImageCollection('LANDSAT/LC08/C01/T1_TOA')
  .filterDate (start,finish)
  .filterBounds(pt)
  .filter("WRS_ROW <= 122")
  .map(maskL8);

var myB8 = l8.select("B8");

//receiving the values at pt for each image

var getB8 = function(image) {

  // Reducing region and getting value
  var value_B8 = ee.Image(image)
    .reduceRegion(ee.Reducer.first(), pt)
    .get('B8');

  return value_B8;
};

var count = myB8.size();

var listOfImages = l8.toList(count);

var B8_list = listOfImages.map(getB8);

print("B8 list", B8_list);

var nonNulls = B8_list.filter(ee.Filter.neq('item', null));

print("Non Null Values", nonNulls);

var idxList = nonNulls.map(function extract (ele) {
  
  var idx1 = nonNulls.indexOf(ele);
  
  var idx2 = B8_list.indexOf(nonNulls.get(idx1)); 
  
  return idx2;
  
});

print("Indices of Non Null Values", idxList);

//Ratio function

var RatioALL = idxList.map(function  calculateRatio (ele) {
  
  var idx = ele;

  var value = ee.Number(B8_list.get(idx));
  
  var image = listOfImages.get(idx);
  
  var ratio1 = ee.Image(image).select('B8')
                              .clip(polygon)
                              .divide(value);

  var time = ee.Image(image).get('system:time_start');

  return ratio1.set('system:time_start', time);
  
});

print("Original Image Collection", myB8);

print ("Ratio All for Non Null Values", RatioALL);