Decrease image size.

Dockerfile optimization leading to final image size reduction.

Before:
```bash
REPOSITORY               TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
hazelcast-k8s            latest              907616f1ae96        2 minutes ago       981.3 MB
```

After:
```bash
REPOSITORY               TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
hazelcast-k8s            latest              4730c4ecf1f0        1 minutes ago       782.8 MB
```

Signed-off-by: Tiago Pires <tandrepires@gmail.com>