docker_compose(role): add support for deploying optional .env file

This is needed for situations, where one wants to use a vendor-provided
compose file and configure it using environment variables.
Like for example:
https://github.com/zammad/zammad-docker-compose

roles/docker_compose/meta/argument_specs.yaml

@@ -7,6 +7,12 @@ argument_specs:
           `/ansible_docker_compose/compose.yaml`.
         type: str
         required: true
+      docker_compose__env_file_content:
+        description: >-
+          The content of the .env file at
+          `/ansible_docker_compose/.env`.
+        type: str
+        required: false
       docker_compose__configuration_files:
         description: >-
           A list of configuration files to be deployed in the