[feat] docker: add DEBUG_ENTRYPOINT variable

Boolean variable to enable `set -x` in the entrypoint script.

release/docker/docker-entrypoint.sh

 #!/bin/sh
 set -u
 
-home=/var/lib/duniter
-config=/etc/duniter
-home_default=$home/duniter_default
-
 function boolean () {
   echo "$1" | sed -E 's/^(true|yes|1)$/true/i'
 }
 
+DEBUG_ENTRYPOINT="$(boolean "${DEBUG_ENTRYPOINT:-false}")"
+if [ "$DEBUG_ENTRYPOINT" = true ]; then
+  set -x
+fi
+
+home=/var/lib/duniter
+config=/etc/duniter
+home_default=$home/duniter_default
+
 manual_config="$(boolean "${DUNITER_MANUAL_CONFIG:-false}")"
 auto_sync="$(boolean "${DUNITER_AUTO_SYNC:-false}")"