Delay Dendrite startup so that matrix-postgres.service can start

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

@@ -16,6 +16,12 @@ Environment="HOME={{ matrix_systemd_unit_home_path }}"
 ExecStartPre=-{{ matrix_host_command_docker }} kill matrix-dendrite
 ExecStartPre=-{{ matrix_host_command_docker }} rm matrix-dendrite
 
+{% if 'matrix-postgres.service' in matrix_dendrite_systemd_required_services_list %}
+# Dendrite is too quick to start in relation to its matrix-postgres dependency.
+# Delay Dendrite startup to avoid failing with: "failed to connect to accounts db" ("pq: the database system is starting up").
+ExecStartPre={{ matrix_host_command_sleep }} 5
+{% endif %}
+
 ExecStart={{ matrix_host_command_docker }} run --rm --name matrix-dendrite \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \