What is a "container" in the context of the following text?:
5.1. Persistence Contexts
A persistence context is a sort of cache; we sometimes call it the "first-level cache", to distinguish it from the second-level cache. For every entity instance read from the database within the scope of a persistence context, and for every new entity made persistent within the scope of the persistence context, the context holds a unique mapping from the identifier of the entity instance to the instance itself.

Thus, an entity instance may be in one of three states with respect to a given persistence context:

transient — never persistent, and not associated with the persistence context,

persistent — currently associated with the persistence context, or

detached — previously persistent in another session, but not currently associated with this persistence context.

Entity lifecycle
At any given moment, an instance may be associated with at most one persistence context.

The lifetime of a persistence context usually corresponds to the lifetime of a transaction, though it’s possible to have a persistence context that spans several database-level transactions that form a single logical unit of work.

A persistence context—that is, a Session or EntityManager—absolutely positively must not be shared between multiple threads or between concurrent transactions.

If you accidentally leak a session across threads, you will suffer.

Container-managed persistence contexts
In a container environment, the lifecycle of a persistence context scoped to the transaction will usually be managed for you.

There are several reasons we like persistence contexts.

They help avoid data aliasing: if we modify an entity in one section of code, then other code executing within the same persistence context will see our modification.

They enable automatic dirty checking: after modifying an entity, we don’t need to perform any explicit operation to ask Hibernate to propagate that change back to the database. Instead, the change will be automatically synchronized with the database when the session is flushed.

They can improve performance by avoiding a trip to the database when a given entity instance is requested repeatedly in a given unit of work.

They make it possible to transparently batch together multiple database operations.

A persistence context also allows us to detect circularities when performing operations on graphs of entities. (Even in a stateless session, we need some sort of temporary cache of the entity instances we’ve visited while executing a query.)

On the other hand, stateful sessions come with some very important restrictions, since:

persistence contexts aren’t threadsafe, and can’t be shared across threads, and

a persistence context can’t be reused across unrelated transactions, since that would break the isolation and atomicity of the transactions.

Furthermore, a persistence context holds a hard references to all its entities, preventing them from being garbage collected. Thus, the session must be discarded once a unit of work is complete.

If you don’t completely understand the previous passage, go back and re-read it until you do. A great deal of human suffering has resulted from users mismanaging the lifecycle of the Hibernate Session or JPA EntityManager.

We’ll conclude by noting that whether a persistence context helps or harms the performance of a given unit of work depends greatly on the nature of the unit of work. For this reason Hibernate provides both stateful and stateless sessions.