gradlew missing executable acl

The `gradlew` file is missing the executable bit.

```bash
-rw-rw-r--
```
should be:
```bash
-rwxrwxr-x
```
https://github.com/openremote/custom-project/blob/9479eb19cebe0284027dfd7bf57bbdf6b245c0cc/docker-compose.yml#L15

Otherwise this 'guide' can't be followed by copy-pasting.