Fix GitLab CI error

Due to the incomplete entrypoint, bash doesn't want to execute scripts.
The error message it provides says:

```
/bin/sh: /bin/sh: cannot execute binary file
```

The solution is rather trivial by making the entrypoint `["/bin/bash",
"-c"]`.

.gitlab-ci.yml

@@ -2,6 +2,7 @@ image:
   name: docker.io/renovate/renovate:19
   entrypoint: 
     - "/bin/bash"
+    - "-c"
 
 stages:
   - CI