Find selectors with-in context of container

I'm using this for a CKEditor plugin and I couldn't for the life of me figure out why I couldn't get this plugin to work. It turns out that the following didn't have any context for where these elements were:
```js
var $all = $(selector)
```
I have the plugin loaded on the actual page (that has jQuery and the plugin). The CKEditor DOM instance (iframe) falls outside the scope of the jQuery document, however. So when I was selecting my elements, it couldn't find the ones inside my CKEditor instance; it would only search the parent DOM. Changing it to the following fixes this:
```js
var $all = $container.find(selector)
```