Bump nginx from 5b44a5a to d6f8c6b in /docker-nginx

Bumps nginx from `5b44a5a` to `d6f8c6b`.

---
updated-dependencies:
- dependency-name: nginx
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

docker-nginx/Dockerfile

@@ -1,4 +1,4 @@
 # 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@sha256:5b44a5ab8ab467854f2bf7b835a32f850f32eb414b749fbf7ed506b139cd8d6b
+FROM nginx:stable-alpine@sha256:d6f8c6b38edf2b7c6619718ad7a00caa4ed4741d12ce12550333a28d2b52dc89