How to locate a Magento 2 installation folder on an unknown server?

find / -name "ISSUE_TEMPLATE.md" -print 2>/dev/null

ISSUE_TEMPLATE.md is a file in the Magento 2 root folder.

Magento 2 installation could have another (duplicate) ISSUE_TEMPLATE.md file in the vendor/magento/magento2-base subfolder, just ignore it.