Testing needs compilation and is slow. Format checking is quick.
As format checking fails more often than tests,
switching them should result in faster failure and feedback
Setting cargo to run incremental builds means partial build results should be cached. This is not enabled by default in release mode.
Incremental builds use 256 codegen units by default [1].
We set them to 16 (release default) again for somewhat faster code but slightly slower builds.
[1]: https://doc.rust-lang.org/cargo/reference/profiles.html#codegen-units
By default, jobs without tags only run on CI runners configured to do so [1].
Conduit can use famedly runners, which are more powerfull than gitlab's runners, but require a tag on the job to run it there.
This commit tags each job with the "docker" tag.
On the famedly/conduit repo this means faster CI.
On other gitlab.com forks the normal ci.
Selfhosted gitlab's might need to add a "docker" tag to their runner.
[1]: https://docs.gitlab.com/ee/ci/runners/configure_runners.html#use-tags-to-limit-the-number-of-jobs-using-the-runner
