Fix docker provider add_fields processors (#1420)

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 commig fixes it.

Fixes elastic/beats#29030

(cherry picked from commit 2223138)

Author: belimawr

changelog/fragments/1664989867-fix-docker-provider-processors.yaml

@@ -0,0 +1,31 @@
+# Kind can be one of:
+# - breaking-change: a change to previously-documented behavior
+# - deprecation: functionality that is being removed in a later release
+# - bug-fix: fixes a problem in a previous version
+# - enhancement: extends functionality but does not break or fix existing behavior
+# - feature: new functionality
+# - known-issue: problems that we are aware of in a given version
+# - security: impacts on the security of a product or a user’s deployment.
+# - upgrade: important information for someone upgrading from a prior version
+# - other: does not fit into any of the other categories
+kind: bug-fix
+
+# Change summary; a 80ish characters long description of the change.
+summary: Fix docker provider add_fields processors
+
+# Long description; in case the summary is not enough to describe the change
+# this field accommodate a description without length limits.
+#description:
+
+# Affected component; a word indicating the component this changeset affects.
+component: providers
+
+# PR number; optional; the PR number that added the changeset.
+# If not present is automatically filled by the tooling finding the PR where this changelog fragment has been added.
+# NOTE: the tooling supports backports, so it's able to fill the original PR number instead of the backport PR number.
+# Please provide it if you are adding a fragment for a different PR.
+#pr: 1234
+
+# Issue number; optional; the GitHub issue related to this changeset (either closes or is part of).
+# If not present is automatically filled by the tooling with the issue linked to the PR number.
+#issue: 1234

internal/pkg/composable/providers/docker/docker.go

@@ -149,7 +149,7 @@ func generateData(event bus.Event) (*dockerContainerData, error) {
 						"image":  container.Image,
 						"labels": processorLabelMap,
 					},
-					"to": "container",
+					"target": "container",
 				},
 			},
 		},

internal/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",
 			},
 		},
 	}