Bump nginx in /{{ cookiecutter.project_name }}/docker/dist/nginx

Bumps nginx from 1.15.3-alpine to 1.21.1-alpine.

---
updated-dependencies:
- dependency-name: nginx
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

{{ cookiecutter.project_name }}/docker/dist/nginx/Dockerfile

@@ -1,4 +1,4 @@
-FROM nginx:1.15.3-alpine
+FROM nginx:1.21.1-alpine
 WORKDIR /app
 RUN mkdir /app/statics