how to sort an entity’s arrayCollection in symfony2

You should be able to use the @ORM\OrderBy statement which allows you to specify columns to order collections on:

/**
 * @ORM\OneToMany(targetEntity="BizTV\ContentManagementBundle\Entity\Content", mappedBy="container")
 * @ORM\OrderBy({"sort_order" = "ASC"})
 */
private $content;

In fact this may be a duplicate of How to OrderBy on OneToMany/ManyToOne

Edit

Checking for implementation advice it appears that you must fetch the tables with a join query to the collection in order for the @ORM\OrderBy annotation to work: http://www.krueckeberg.org/notes/d2.html

This means that you must write a method in the repository to return the container with the contents table joined.