Read all the comments first - there are a number of corrections and important points in the comment section
This is a summarized guide I created while installing Firefly-III in a Proxmox (6.2-15) container running Ubuntu 20.04.
I followed this tutorial. It has a link to this other tutorial for installing mariaDB. Furthermore, the official Firefly-iii documentation is Here .
This guide has everything I did in one concise summary. Not much is explained. This will take about 1.5 hrs.
Note: I initially followed the official instructions and used a LAMP stack. I had issues with apache and getting the webserver to work (likely because I am not familiar with apache and am more comfortable with nginx).