The whole CI is refactored. The following problems are solved:

1. Couldn't test base image locally without overriding the old one.

The testing stage is run in two ways depending on whether some
of the docker-related files were changed.

If they didn't (most likely), testing stage is run on the `latest`
docker image as it used to.

Otherwise, the docker image is rebuilt locally and tested. This is done
by using `image:pull_policy` option, see [1].

It also makes a problem of using protected credentials irrelevant.

2. Gitlab caching used to be slow

Stupid gitlab caching (see [2]) wasted on (un)zipping cache 4 minutes
(of 6 total). Now it's replaced with a manual one: an archive (tar) is
placed in a docker volume on a self-hosted gitlab runner with no
compression.

3. Speed up fetching git.

The tarantool-sys submodule requires a tag to be built correctly, but
using shallow clones prevented that. We used to work-around it by
specifying `GIT_DEPTH: 0`, but that's sub-optimal too. This patch
introduces an elegant yet reliable solution:

```
until git describe; do git fetch --deepen 100; done
```

4. No fine-grained time tracking was available

Gitlab only shows the total time of `step_script` without splitting it
into particular commands. This problem is solved by incorporating
collapsible log sections feature, see [3]. It makes logs fancy too.

Refs:

- [1] https://docs.gitlab.com/ee/ci/yaml/#imagepull_policy
- [2] https://gitlab.com/gitlab-org/gitlab-runner/-/issues/3523
- [3] https://docs.gitlab.com/ee/ci/jobs/#custom-collapsible-sections