Why does my updated Debian buster server with only Docker services running keep rebooting randomly multiple times a day?

I have an updated Debian Buster server hosted in my friend house, it used to serve websites under Docker.

There's only docker running on it, with the following services:

• ghost:3.36.0-alpine (x2)
• registry.gitlab.com/comment/commento:v1.8.0
• postgres:13.0-alpine
• traefik:v2
• pajikos/sms-gammu-gateway:latest

This is my OS information:

Linux master-letort-01 5.8.0-0.bpo.2-amd64 #1 SMP Debian 5.8.10-1~bpo10+1 (2020-09-26) x86_64 GNU/LinuxNo LSB modules are available.Distributor ID: DebianDescription:    Debian GNU/Linux 10 (buster)Release:    10Codename:   buster

This is the result of:

• lshw (hardware info): http://paste.debian.net/1169348/
• lsmod (driver info): https://termbin.com/od1lo
• sensors (temperature info): http://paste.debian.net/1169353/
• last reboot (reboot info): http://paste.debian.net/1169354/

It reboot randomly from 1 to 8 times a day, each time causing a downtime of 6 minutes, causing a total of a maximum of 1h down per day.

I have checked /var/log/syslog and dmesg, I do not see any failure, see the logs:

• /var/log/syslog: : http://paste.debian.net/1169349/
• dmesg: http://paste.debian.net/1169350/

I have enabled the persistent journal logging yesterday with :

mkdir -p /var/log/journal; systemd-tmpfiles --create --prefix /var/log/journal;

This is the result of journalctl --boot=-1 --pager-end after the last crash : https://termbin.com/32xm

I can see the following in red in dmesg:

[    2.236521] i915 0000:00:02.0: firmware: failed to load i915/kbl_dmc_ver1_04.bin (-2)[    2.236525] firmware_class: See https://wiki.debian.org/Firmware for information about missing firmware

I think it's not related at all, I have now installed firmware-misc-nonfree, I believe this will remove the only error I have found on it.

Since I am not able to find any logs, I am about to order a new power supply unit. I am not sure if this is the right decision as the whole computer is new but not under warranty.

1. Why does my updated Debian buster server with only Docker services running keep rebooting randomly multiple times a day?
2. What are my best options/ways to troubleshoot and find the cause ?
3. What are my chances to fix it by replacing the PSU (Power supply unit) ?