Race condition at first time startup

When containers are started the first time, there might occure a dace condition while the database is not yet fully set up. database_ready seams not to return a vaild integer value then.

```
ttrss_php_1 | Initializing config.php
ttrss_php_1 | /docker-entrypoint.sh: line 49: [: database_ready: integer expression expected
```
Proposed fix: within the function check the value for 1, otherwise return 0 instead of returning the variable