container: make dockerfile variable officially supported and more modular

roles/container/meta/argument_specs.yml

@@ -17,6 +17,12 @@ argument_specs:
         description: "Docker image to use for the container. If dockerfile is defined, it will be used as base for locally built image (example: gitea/gitea:latest)"
         type: str
         required: true
+      dockerfile:
+        description: "A list of dockerfile instructions to add to the base image"
+        type: list
+        elements: str
+        required: false
+        default: []
       docker_host_user:
         description: "If true, creates a user on the host for this service. The container will run as this user's uid/gid. Bind mount volumes will be owned by this user."
         type: bool