Problem

When you are trying to install a package inside a debian Docker container you get the following error message:

FATAL -> Failed to fork.

Solution

This is an indication of ‘out of memory’ so add a swap partition if it does not exist, on the host with the following:

fallocate -l 4G /swapfile
chmod 600 /swapfile
mkswap /swapfile
swapon /swapfile
echo "/swapfile swap swap sw 0 0" >> /etc/fstab

(Thanks Valent)