Fix WORK_DIR for docker (root) image (go-gitea#25738) (go-gitea#25811)

Backport go-gitea#25738 by @wxiaoguang

Fix go-gitea#25726 

go-gitea#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: wxiaoguang <wxiaoguang@gmail.com>

docker/root/usr/local/bin/gitea

@@ -8,7 +8,7 @@
 #
 # And place the original in /usr/lib/gitea with working files in /data/gitea
 GITEA="/app/gitea/gitea"
-WORK_DIR="/app/gitea"
+WORK_DIR="/data/gitea"
 CUSTOM_PATH="/data/gitea"
 
 # Provide docker defaults