Bump mongodb from 3.6.3 to 3.6.4 in /docker/mqtt-mongo

Bumps [mongodb](https://github.com/mongodb/node-mongodb-native) from 3.6.3 to 3.6.4.
- [Release notes](https://github.com/mongodb/node-mongodb-native/releases)
- [Changelog](https://github.com/mongodb/node-mongodb-native/blob/3.6/CHANGES_3.0.0.md)
- [Commits](mongodb/node-mongodb-native@v3.6.3...v3.6.4)

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