Add thread-stacksize = 512 to uwsgi config

When running in staging without this set we would see workers randomly
die causing 502s
`DAMN ! worker 5 (pid: 429) died, killed by signal 11 :( trying respawn
...`
unbit/uwsgi#1792 (comment)

docker/web/conf/uwsgi.ini

@@ -29,6 +29,10 @@ stats = /tmp/uwsgistats.sock
 
 threads = 2
 enable-threads = true
+# stop workers dying
+# DAMN ! worker 5 (pid: 429) died, killed by signal 11 :( trying respawn ...
+# https://github.com/unbit/uwsgi/issues/1792#issuecomment-481755089
+thread-stacksize = 512
 
 thunder-lock = true
 vacuum = true