Backport #25738 by @wxiaoguang
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
```
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This PR is breaking but this is the only way at the moment to avoid
users losing their data accidently
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
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