How to use a custom DbContext to implement a Repository in Entity Framework?

At work, we followed a routine that looked like this:

interface ICRUD<T> : ICreatable<T>, IRetrievable<T>, IUpdatable<T>, IDeletable<T>
{
}

interface ICreatable<T>
{
    T Create();
}

interface IRetrieve<T>
{
    T Retrieve(params object[] keys);
}

interface IUpdatable<T>
{
    void Update(T existing);
}

interface ICreatable<T>
{
    void Delete(T existing);
}

Then we developed a basic repository driven by Entities:

public abstract class BaseRepository<TModel, TEntities> where TEntities : IDbSet<TModel>
{
    protected TEntities Entities {get; set;}
    protected DbContext Db {get; set;}

    public BaseRepository (DbContext db, TEntities entities)
    {
        Db = db;
        Entities = entities;
    }

    public virtual TModel Create() { return Entities.Create (); }
    public virtual TModel Retrieve (params object[] keys) { return Entities.Find (keys); }
    public virtual void Update (TModel existing) { Db.Entry(existing).State = Modified; }
    public virtual void Delete (TModel existing) { Db.Entry(existing).State = Removed; }
}

If you'll see, the BaseRepository just shares method signatures and doesn't really utilize ICRUD. Since we write our code according to interfaces, we can utilize a lot of common code without giving base classes access to capabilities we don't want them to have. Without having any prior knowledge of Entities, a developer is free to implement the data store however they see fit (ICRUD can communicate with a web service, for example), or they are free to extend the functionality offered by the BaseRepository by overriding any of the available methods and performing their own unique actions.