fix: s6

Dockerfile

@@ -8,6 +8,7 @@ LABEL description="A complete, self-hosted Tiny Tiny RSS (TTRSS) environment." \
 RUN set -xe && \
     apk update && apk upgrade && \
     apk add --no-cache --virtual=run-deps \
+    s6 s6-linux-init s6-overlay \
     busybox nginx git ca-certificates curl \
     php7 php7-fpm php7-curl php7-dom php7-gd php7-iconv php7-fileinfo php7-json \
     php7-mcrypt php7-pgsql php7-pcntl php7-pdo php7-pdo_pgsql \