Allow 'git' as a version for s3 storage provider

roles/custom/matrix-synapse/templates/synapse/customizations/Dockerfile.j2

@@ -1,7 +1,11 @@
 FROM {{ matrix_synapse_docker_image }}
 
 {% if matrix_synapse_container_image_customizations_s3_storage_provider_installation_enabled %}
+    {% if matrix_synapse_ext_synapse_s3_storage_provider_version == 'git' %}
+RUN apt-get update -yq && apt-get install -yq git && pip install git+https://github.com/matrix-org/synapse-s3-storage-provider.git
+    {% else %}
 RUN pip install synapse-s3-storage-provider=={{ matrix_synapse_ext_synapse_s3_storage_provider_version }}
+    {% endif %}
 {% endif %}
 
 {{ matrix_synapse_container_image_customizations_dockerfile_body_custom }}