Fix s6-svscan path in Dockerfile (#7867)
Related to #7864
## Describe the pull request
Fix the path of the s6-svscan binary path in the Dockerfile. As
mentionned in the issue, it is probably to the alpine base image change
(from `3.17` to `3.21`).
Link to the issue:
https://github.com/gogs/gogs/issues/7864#issuecomment-2558323928
Credit to @cryptovaltt
## Test plan
Build the image and check if the application can be ran. E.g.:
```bash
docker build -t gogs:working-7864 -f Dockerfile .
docker run --name=gogs -p 10022:22 -p 10880:3000 -v gogs:working-7864 --rm
```
Alexandre Jacquin