Standardize image naming

Agreed upon naming strategy is `project-type-os` where type is `project` is `all|rl|vw`, `type` is `build|dev`, and os is `ubuntu-1604|...`


- [ ] vowpalwabbit/ubuntu1604-build
- [ ] vowpalwabbit/ubuntu1404-build
- [ ] vowpalwabbit/centos7_6_1810-build
- [ ] vowpalwabbit/rl-ubuntu-1604
- [x] vowpalwabbit/all-dev-ubuntu1604

