dhi: add customization working directory (#23973)

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## Description

Added new working directory override option to image customization.

## Related issues or tickets

https://docker.slack.com/archives/C04M34MRQS1/p1768231979495499

## Reviews

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- [ ] Editorial review

Signed-off-by: Craig Osterhout <craig.osterhout@docker.com>

content/manuals/dhi/how-to/customize.md

@@ -110,6 +110,8 @@ To customize a Docker Hardened Image, follow these steps:
       image. These arguments are appended to the base image's entrypoint.
    1. Add [`CMD`](/reference/dockerfile/#cmd) arguments to the image. These
       arguments are appended to the base image's command.
+   1. Override the default (`/`) [working
+      directory](/reference/dockerfile/#workdir) for the image.
    1. Specify a suffix for the customization name that is appended to the
       customized image's tag. For example, if you specify `custom` when
       customizing the `dhi-python:3.13` image, the customized image will be