Prefer --mount instead of -v for mounting volumes

This doesn't replace all usage of `-v`, but it's a start.

People sometimes troubleshoot by deleting files (especially bridge
config files). Restarting Synapse with a missing registration.yaml file
for a given bridge, causes the `-v
/something/registration.yaml:/something/registration.yaml:ro` option
to force-create `/something/registration.yaml` as a directory.

When a path that's provided to the `-v` option is missing, Docker
auto-creates that path as a directory.
This causes more breakage and confusion later on.

We'd rather fail, instead of magically creating directories.
Using `--mount`, instead of `-v` is the solution to this.

From Docker's documentation:

> When you use --mount with type=bind, the host-path must refer to an existing path on the host.
> The path will not be created for you and the service will fail with an error if the path does not exist.

roles/matrix-synapse/tasks/ext/rest-auth/setup_install.yml

@@ -17,13 +17,11 @@
 - set_fact:
     matrix_synapse_password_providers_enabled: true
 
-- set_fact:
-    matrix_synapse_container_additional_volumes: >
-      {{ matrix_synapse_container_additional_volumes }}
+    matrix_synapse_container_extra_arguments: >
+      {{ matrix_synapse_container_extra_arguments|default([]) }}
       +
-      {{ [{'src': '{{ matrix_synapse_ext_path }}/rest_auth_provider.py', 'dst': '{{ matrix_synapse_in_container_python_packages_path }}/rest_auth_provider.py', 'options': 'ro'}] }}
+      {{ ["--mount type=bind,src={{ matrix_synapse_ext_path }}/rest_auth_provider.py,dst={{ matrix_synapse_in_container_python_packages_path }}/rest_auth_provider.py,ro"] }}
 
-- set_fact:
     matrix_synapse_additional_loggers: >
       {{ matrix_synapse_additional_loggers }}
       +