Just found out that systemd-networkd was enabled by default. Disabling it either by running systemctl disable systemd-networkd inside the container or by deleting /etc/systemd/system/socket.target.wants/systemd-networkd.socket worked both fine: Now the container keeps the IP address assigned via its config file.
However, I still do still not understand how assigning the IP defined inside the config file to the containers network interface works in the first place. Would be nice if one could point me to some documentation about this magic 