Добавил копирование renewal.sh в образ.

Dockerfile

@@ -1,6 +1,8 @@
 FROM centos:7.4.1708
 COPY nginx.repo /etc/yum.repos.d/nginx.repo
-COPY generate.sh /tmp
+COPY generate.sh \
+     renewal.sh \
+     /tmp
 
 RUN yum install -y epel-release \
     && yum install -y yum-utils python2-certbot-nginx openssl ca-certificates mailx nginx \
@@ -12,6 +14,7 @@ RUN yum install -y epel-release \
     && mkdir -p /var/log/letsencrypt \
     && touch /var/log/letsencrypt/letsencrypt.log \
     && chmod +x /tmp/generate.sh \
+    && chmod +x /tmp/renewal.sh \
     && ln -sf /dev/stdout /var/log/letsencrypt/letsencrypt.log \
     && ln -sf /dev/stdout /var/log/nginx/access.log \
     && ln -sf /dev/stderr /var/log/nginx/error.log