docs: fix misleading example of setting an env variable for a single command

The `DEBIAN_FRONTEND` environment variable is used to control the interface by which debconf questions are presented to the user (see [`man 7 debconf`][1]). In `DEBIAN_FRONTEND=noninteractive apt-get update && apt-get install -y`, the `DEBIAN_FRONTEND` environment variable is only set for the `apt-get update` command which does not ask debconf questions, and will not affect the `apt-get install` command where these questions are actually asked. It should be the other way around.

  [1]: https://manpages.debian.org/debconf.7.html

Signed-off-by: Murukesh Mohanan <[email protected]>
(cherry picked from commit 7227c01)
Signed-off-by: Sebastiaan van Stijn <[email protected]>

Author: muru

docs/reference/builder.md

@@ -1060,7 +1060,7 @@ If an environment variable is only needed during build, and not in the final
 image, consider setting a value for a single command instead:
 
 ```dockerfile
-RUN DEBIAN_FRONTEND=noninteractive apt-get update && apt-get install -y ...
+RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y ...
 ```
  
 Or using [`ARG`](#arg), which is not persisted in the final image: