settings/production: add NUM_PROXIES override for varnish

Close: T884

swh/web/settings/production.py

@@ -5,6 +5,7 @@
 
 from .common import *  # noqa
 from .common import swh_web_config
+from .common import REST_FRAMEWORK
 
 # activate per-site caching
 MIDDLEWARE += ['django.middleware.cache.UpdateCacheMiddleware', # noqa
@@ -23,3 +24,6 @@ USE_X_FORWARDED_HOST = True
 SECURE_PROXY_SSL_HEADER = ('HTTP_X_FORWARDED_PROTO', 'https')
 
 ALLOWED_HOSTS += ['archive.softwareheritage.org']  # noqa
+
+# We're going through seven (or, in that case, 2) proxies thanks to Varnish
+REST_FRAMEWORK['NUM_PROXIES'] = 2