Added an env var in the Dockerfile

Regarding the start.sh file, to run Horizon, this env variable must exist !

contrib/docker/Dockerfile.apache

@@ -54,6 +54,7 @@ ENV APP_ENV=production \
     DB_HOST=db \
     BROADCAST_DRIVER=log \
     QUEUE_DRIVER=redis \
+    HORIZON_EMBED=TRUE \
     HORIZON_PREFIX=horizon-pixelfed \
     REDIS_HOST=redis \
     SESSION_SECURE_COOKIE=true \