Make docker-entrypoint.sh executable to avoid error

Was getting an error when trying to run the image:

```
$ docker run --rm robisenberg/redis
container_linux.go:265: starting container process caused "exec: \"docker-entrypoint.sh\": executable file not found in $PATH"
docker: Error response from daemon: oci runtime error: container_linux.go:265: starting container process caused "exec: \"docker-entrypoint.sh\": executable file not found in $PATH".
```

The fix seemed to be to make the docker-entrypoint.sh executable.
See moby/moby#27182

Dockerfile

@@ -71,6 +71,7 @@ VOLUME /data
 WORKDIR /data
 
 COPY docker-entrypoint.sh /usr/local/bin/
+RUN chmod +x /usr/local/bin/docker-entrypoint.sh
 ENTRYPOINT ["docker-entrypoint.sh"]
 
 CMD ["redis-server"]