Add standalone DAG processor to docker compose (apache#45650)

We are moving to only support a standalone DAG processor in Airflow 3,
so let's switch our compose setup to have it. We can remove the env var
once that's the only option, but for now it let's us avoid a massive PR
:)

docs/apache-airflow/howto/docker-compose/docker-compose.yaml

@@ -60,6 +60,7 @@ x-airflow-common:
     AIRFLOW__CORE__FERNET_KEY: ''
     AIRFLOW__CORE__DAGS_ARE_PAUSED_AT_CREATION: 'true'
     AIRFLOW__CORE__LOAD_EXAMPLES: 'true'
+    AIRFLOW__SCHEDULER__STANDALONE_DAG_PROCESSOR: 'true'
     AIRFLOW__API__AUTH_BACKENDS: >-
        airflow.providers.fab.auth_manager.api.auth.backend.basic_auth,airflow.api.auth.backend.session
     # yamllint disable rule:line-length
@@ -148,6 +149,21 @@ services:
       airflow-init:
         condition: service_completed_successfully
 
+  airflow-dag-processor:
+    <<: *airflow-common
+    command: dag-processor
+    healthcheck:
+      test: ["CMD-SHELL", 'airflow jobs check --job-type DagProcessorJob --hostname "$${HOSTNAME}"']
+      interval: 30s
+      timeout: 10s
+      retries: 5
+      start_period: 30s
+    restart: always
+    depends_on:
+      <<: *airflow-common-depends-on
+      airflow-init:
+        condition: service_completed_successfully
+
   airflow-worker:
     <<: *airflow-common
     command: celery worker

docs/apache-airflow/howto/docker-compose/index.rst

@@ -88,6 +88,7 @@ This file contains several service definitions:
 
 - ``airflow-scheduler`` - The :doc:`scheduler </administration-and-deployment/scheduler>` monitors all tasks and DAGs, then triggers the
   task instances once their dependencies are complete.
+- ``airflow-dag-processor`` - The DAG processor parses DAG files.
 - ``airflow-webserver`` - The webserver is available at ``http://localhost:8080``.
 - ``airflow-worker`` - The worker that executes the tasks given by the scheduler.
 - ``airflow-triggerer`` - The triggerer runs an event loop for deferrable tasks.