[misc] apply review feedback

- move setting into clsi.docker namespace
- rename the variable for images to allowedImages / ALLOWED_IMAGES
- add an additional check for the image name into the DockerRunner

Co-Authored-By: Brian Gough <brian.gough@overleaf.com>

test/unit/js/CompileControllerTests.js

@@ -306,9 +306,10 @@ describe('CompileController', function() {
         .should.equal(true)
     })
 
-    describe('when allowedImageNamesFlat is set', function() {
+    describe('when allowedImages is set', function() {
       beforeEach(function() {
-        this.Settings.allowedImageNamesFlat = [
+        this.Settings.clsi = { docker: {} }
+        this.Settings.clsi.docker.allowedImages = [
           'repo/image:tag1',
           'repo/image:tag2'
         ]