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
