Fix docker provider add_fields processors

The Docker provider was using a wrong key when defining the
`add_fields` processor, this causes Filebeat not to start the input
and stay on a unhealthy state. This processor seems to be defined
every time a variable like `${docker}` is used on a input
configuration.

This commig fixes it.

x-pack/elastic-agent/CHANGELOG.next.asciidoc

@@ -96,6 +96,7 @@
 - Allow ':' characters in dynamic variables. {pull}32407[32407]
 - Allow the - char to appear as part of variable names in eql expressions. {pull}32350[32350]
 - Allow the / char to appear as part of variable names in eql expressions. {pull}32528{32528}
+- Fix add_fields processor on Docker provider {pull}33269{33269}
 
 ==== New features
 

x-pack/elastic-agent/pkg/composable/providers/docker/docker.go

@@ -145,7 +145,7 @@ func generateData(event bus.Event) (*dockerContainerData, error) {
 						"image":  container.Image,
 						"labels": processorLabelMap,
 					},
-					"to": "container",
+					"target": "container",
 				},
 			},
 		},

x-pack/elastic-agent/pkg/composable/providers/docker/docker_test.go

@@ -53,7 +53,7 @@ func TestGenerateData(t *testing.T) {
 						"co_elastic_logs/disable": "true",
 					},
 				},
-				"to": "container",
+				"target": "container",
 			},
 		},
 	}