make synapse support alternative containers via new variable

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

@@ -70,8 +70,9 @@ matrix_synapse_container_image_customizations_templates_git_repository_keyscan_h
 #  RUN echo 'You do NOT need to include a FROM clause yourself.'
 matrix_synapse_container_image_customizations_dockerfile_body_custom: ''
 
-matrix_synapse_docker_image: "{{ matrix_synapse_docker_image_name_prefix }}matrix-org/synapse:{{ matrix_synapse_docker_image_tag }}"
+matrix_synapse_docker_image: "{{ matrix_synapse_docker_image_name_prefix }}{{ matrix_synapse_docker_image_name }}:{{ matrix_synapse_docker_image_tag }}"
 matrix_synapse_docker_image_name_prefix: "{{ 'localhost/' if matrix_synapse_container_image_self_build else matrix_synapse_docker_image_registry_prefix }}"
+matrix_synapse_docker_image_name: matrix-org/synapse
 matrix_synapse_docker_image_tag: "{{ matrix_synapse_version }}"
 matrix_synapse_docker_image_force_pull: "{{ matrix_synapse_docker_image.endswith(':latest') }}"
 matrix_synapse_docker_image_registry_prefix: ghcr.io/