fix: gracefully exit on sigterm

The purpose is to avoid duniter database corruption on docker stop.

Before that sigterm was ignored, causing docker instance being force-killed on docker stop command after the related timeout expiration.

Exerpt from the docker stop manpage:

The main process inside the container will receive SIGTERM, and after a grace period, SIGKILL

release/docker/docker-entrypoint.sh

 #!/bin/sh
 set -uo pipefail
 
-function boolean () {
+boolean () {
   echo "$1" | sed -E 's/^(true|yes|1)$/true/i'
 }
 big_fat_warning='\033[01;31m**WARNING**:\033[0m'