* Upgrade docker and docker:dind images to 20.10
* Use docker-buildx image maintained by @jarvis-network [1] [2]
* Enable TLS communication with Docker engine
* Remove unnecessary variable settings
* Remove unused deploy stage
* Extract common logic from `before_script` and make it more generic
* Express all final build targets with a matrix
* Rename the stages and jobs names to be more descriptive

[1]: https://gitlab.com/jarvis-network/base/container-images/docker-buildx
[2]: registry.gitlab.com/jarvis-network/base/container-images/docker-buildx:20.10