Fix docker warning in `Containerfile`

Relates to: https://docs.docker.com/reference/build-checks/from-as-casing/

See: https://github.com/torrust/torrust-index/actions/runs/10141334469/job/28038511583#step:7:19

```
 warnings found (use --debug to expand):
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 6)
```