Improper default proxy_custom_buffer_size for Mattermost
Mattermost wants proxy_custom_buffer_size
of 16k
for location /
per https://github.com/mattermost/mattermost-omnibus/blob/main/mmomni/ansible/playbooks/mattermost.conf & https://docs.mattermost.com/install/config-proxy-nginx.html. Omnibus fails to set this by default.