Use cosmic as nginx env instead of debain/stretch

nginx_cosmic.diff

diff --git a/mainline/stretch/Dockerfile b/mainline/stretch/Dockerfile
index 09e2d84..1f96b37 100644
--- a/mainline/stretch/Dockerfile
+++ b/mainline/stretch/Dockerfile
@@ -1,9 +1,10 @@
-FROM debian:stretch-slim
+FROM ubuntu:cosmic

 LABEL maintainer="NGINX Docker Maintainers <docker-maint@nginx.com>"

-ENV NGINX_VERSION 1.15.8-1~stretch
-ENV NJS_VERSION   1.15.8.0.2.7-1~stretch
+ENV NGINX_VERSION 1.15.8-1~cosmic
+ENV NJS_VERSION   1.15.8.0.2.7-1~cosmic

 RUN set -x \
        && apt-get update \
@@ -33,13 +34,13 @@ RUN set -x \
        && case "$dpkgArch" in \
                amd64|i386) \
 # arches officialy built by upstream
-                       echo "deb https://nginx.org/packages/mainline/debian/ stretch nginx" >> /etc/apt/sources.list.d/nginx.list \
+                       echo "deb https://nginx.org/packages/mainline/ubuntu/ cosmic nginx" >> /etc/apt/sources.list.d/nginx.list \
                        && apt-get update \
                        ;; \
                *) \
 # we're on an architecture upstream doesn't officially build for
 # let's build binaries from the published source packages
-                       echo "deb-src https://nginx.org/packages/mainline/debian/ stretch nginx" >> /etc/apt/sources.list.d/nginx.list \
+                       echo "deb-src https://nginx.org/packages/mainline/ubuntu/ cosmic nginx" >> /etc/apt/sources.list.d/nginx.list \
                        \
 # new directory for storing sources and .deb files
                        && tempDir="$(mktemp -d)" \