Bitbucket Pipelines — GPU support

### Continuation of https://github.com/iterative/cml/pull/798#pullrequestreview-982578801

As per the [official documentation](https://support.atlassian.com/bitbucket-cloud/docs/runners), self-hosted Bitbucket Pipelines runners for Linux don't “provide access to the host file system” although that limitation is “subject to change in the future”

> #### _Linux-only_
>
> _Can’t access the local files on the runner host from a build script: In order to provide isolation and reproducibility, a runner doesn’t provide access directly to the host file system, we recommend using SCP / Rsync, if necessary._
>
> _The limits described above are subject to change in the future._

In practice, this means that every single step is containerized, and users don't have any control over container options like [`docker run --gpus`](https://docs.docker.com/engine/reference/commandline/run/#access-an-nvidia-gpu), even when the NVIDIA runtime is installed.

* https://github.com/iterative/cml/pull/798#issuecomment-1129155399
* https://github.com/iterative/cml/pull/798#issuecomment-1131728075