adds docker image name to docker_image_tag and adds url example

.env-example

 DB_USER=user
 DB_USER_PASSWORD=user
 DOCKER_PREFIX=wp
-DOCKER_IMAGE_TAG=dev
+DOCKER_IMAGE_TAG=wp-php7.3:dev
 MOUNT_USER=username
 TRAEFIK_NETWORK_NAME=traefik
 WP_ADMIN_PASSWORD=admin
 WP_DEFAULT_PROTOCOL=https
 WP_LOCALE=fr_FR
 WP_THEME=twentyseventeen
-WP_URL=
+WP_URL=www.wp.localhost
 WP_VERSION=5.4

docker-compose.yml

       - db:/var/lib/mysql
   web:
     container_name: ${DOCKER_PREFIX}-web
-    image: wp-php7.3:${DOCKER_IMAGE_TAG}
+    image: ${DOCKER_IMAGE_TAG}
     labels:
       - traefik.enable=true
       - traefik.http.routers.${DOCKER_PREFIX}-web.entrypoints=https