Fix WORK_DIR for docker (root) image (#25738)

Fix WORK_DIR for docker (root) image (#25738)

Fix #25726

#17846 chose an incorrect WORK_DIR path for docker root image.

Gitea's work-path was already used as the base path for various paths
(like AppDataPath), so, the work-path should be mounted to a volume in a
docker image.

Now, for docker root image, it's unavoidable to mix the
WorkPath/CustomPath/AppDataPath in the same directory ("/data/gitea"),
because some of them have already been mixed.

Some directories in the screenshot are for "CustomPath" , while others
are for "AppDataPath", due to the technical debts in old code:

```
CUSTOM_PATH="/data/gitea"
APP_DATA_PATH = /data/gitea
```

<details>

![image](https://github.com/go-gitea/gitea/assets/2114189/9f0648ac-f731-4a08-9f26-1af01a1824b1)

</details>

This PR is breaking but this is the only way at the moment to avoid
users losing their data accidently

Co-authored-by: Giteabot <teabot@gitea.io>