Update dockerfile example port (#23448)

## Description

Mismatch between exposed port and port used by command.

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content/get-started/docker-concepts/building-images/writing-a-dockerfile.md

@@ -31,7 +31,7 @@ RUN pip install --no-cache-dir -r requirements.txt
 
 # Copy in the source code
 COPY src ./src
-EXPOSE 5000
+EXPOSE 8080
 
 # Setup an app user so the container doesn't run as the root user
 RUN useradd app