Switch matrix_dendrite_container_additional_volumes from using -v to --mount

Related to e5130372b9.

Depending on the `options` that people provide, this may break
compatibility.

roles/custom/matrix-dendrite/defaults/main.yml

@@ -215,12 +215,8 @@ matrix_dendrite_client_api_recaptcha_sitekey_class: ""
 
 # A list of additional "volumes" to mount in the container.
 # This list gets populated dynamically based on Dendrite extensions that have been enabled.
-# Contains definition objects like this: `{"src": "/outside", "dst": "/inside", "options": "rw|ro|slave|.."}
-#
-# Note: internally, this uses the `-v` flag for mounting the specified volumes.
-# It's better (safer) to use the `--mount` flag for mounting volumes.
-# To use `--mount`, specify it in `matrix_dendrite_container_extra_arguments`.
-# Example: `matrix_dendrite_container_extra_arguments: ['--mount type=bind,src=/outside,dst=/inside,ro']
+# Contains definition objects like this: `{"src": "/outside", "dst": "/inside", "options": "ro"}
+# Note: internally, this uses the `--mount` flag for mounting the specified volumes.
 matrix_dendrite_container_additional_volumes: []
 
 # A list of appservice config files (in-container filesystem paths).

roles/custom/matrix-dendrite/templates/systemd/matrix-dendrite.service.j2

@@ -42,7 +42,7 @@ ExecStartPre={{ devture_systemd_docker_base_host_command_docker }} create \
 			--mount type=bind,src={{ matrix_dendrite_nats_storage_path }},dst=/matrix-nats-store,bind-propagation=slave \
 			--label-file={{ matrix_dendrite_base_path }}/labels \
 			{% for volume in matrix_dendrite_container_additional_volumes %}
-			-v {{ volume.src }}:{{ volume.dst }}:{{ volume.options }} \
+			--mount type={{ volume.type | default('bind' if '/' in volume.src else 'volume') }},src={{ volume.src }},dst={{ volume.dst }}{{ (',' + volume.options) if volume.options else '' }} \
 			{% endfor %}
 			{% for arg in matrix_dendrite_container_arguments %}
 			{{ arg }} \