Remove docker-compose indirection

Dependabot now supports docker-compose files directly, so we don't need
to wrap the upstream nginx image in a Dockerfile for the sake of
dependabot.

.github/dependabot.yml

@@ -1,6 +1,6 @@
 version: 2
 updates:
-  - package-ecosystem: "docker"
-    directory: "/docker-nginx"
+  - package-ecosystem: "docker-compose"
+    directory: "/"
     schedule:
       interval: "weekly"

docker-compose.yml

@@ -1,6 +1,6 @@
 services:
   nginx-terminate:
-    build: ./docker-nginx
+    image: nginx:stable-alpine@sha256:aed99734248e851764f1f2146835ecad42b5f994081fa6631cc5d79240891ec9
     restart: unless-stopped
     volumes:
       - ./data/nginx-terminate:/usr/conf.d
@@ -11,7 +11,7 @@ services:
       - "80:80"
     command: "/bin/sh -c 'while :; do sleep 6h & wait $${!}; /usr/sbin/nginx -s reload; done & /usr/sbin/nginx -c /usr/conf.d/nginx.conf -g \"daemon off;\"'"
   nginx-relay:
-    build: ./docker-nginx
+    image: nginx:stable-alpine@sha256:aed99734248e851764f1f2146835ecad42b5f994081fa6631cc5d79240891ec9
     restart: unless-stopped
     volumes:
       - ./data/nginx-relay:/usr/conf.d

docker-nginx/Dockerfile

@@ -1,4 +0,0 @@
-# This wraps the upstream nginx image so dependabot can watch it for changes.
-# If https://github.com/dependabot/dependabot-core/issues/390 is addressed, we
-# can just inline this into docker-compose.yml
-FROM nginx:stable-alpine@sha256:aed99734248e851764f1f2146835ecad42b5f994081fa6631cc5d79240891ec9