Fix ambiguous flags in Dockerfile example (#33417)

* Fix ambiguous flags in Dockerfile example

At least with Docker xx this Dockerfile fails to both `adduser` and `addgroup` commands due to ambiguous flags. The error message for example for `addgroup` is:

```
#5 0.292 Option g is ambiguous (gecos, gid, group)
#5 0.292 Option s is ambiguous (shell, system)
```
This PR switches both commands to use the long-format flags. I think they are also more understandable for the readers of the Dockerfile.

* Apply suggestions from code review

Co-authored-by: Balázs Orbán <info@balazsorban.com>

Co-authored-by: JJ Kasper <jj@jjsweb.site>
Co-authored-by: Balázs Orbán <info@balazsorban.com>

examples/with-docker/Dockerfile

@@ -23,8 +23,8 @@ WORKDIR /app
 
 ENV NODE_ENV production
 
-RUN addgroup -g 1001 -S nodejs
-RUN adduser -S nextjs -u 1001
+RUN addgroup --system --gid 1001 nodejs
+RUN adduser --system --uid 1001 nextjs
 
 # You only need to copy next.config.js if you are NOT using the default configuration
 # COPY --from=builder /app/next.config.js ./