build: use exec form for flask example

shell form doesnt trap sigint

Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>

content/build/building/packaging.md

@@ -92,7 +92,7 @@ COPY hello.py /
 # final configuration
 ENV FLASK_APP=hello
 EXPOSE 8000
-CMD flask run --host 0.0.0.0 --port 8000
+CMD ["flask", "run", "--host", "0.0.0.0", "--port", "8000"]
 ```
 
 Here's a breakdown of what this Dockerfile does:
@@ -234,12 +234,22 @@ team members understand what this application is doing.
 Finally, [`CMD` instruction](../../engine/reference/builder.md#cmd) sets the
 command that is run when the user starts a container based on this image.
 
+```dockerfile
+CMD ["flask", "run", "--host", "0.0.0.0", "--port", "8000"]
+```
+
+This command starts the flask development server listening on all addresses
+on port `8000`. The example here uses the "exec form" version of `CMD`.
+It's also possible to use the "shell form":
+
 ```dockerfile
 CMD flask run --host 0.0.0.0 --port 8000
 ```
 
-In this case we'll start the flask development server listening on all addresses
+There are subtle differences between these two versions,
-on port `8000`.
+for example in how they trap signals like `SIGTERM` and `SIGKILL`.
+For more information about these differences, see
+[Shell and exec form](../../engine/reference/builder.md#shell-and-exec-form)
 
 ## Building