Check if CI really don't build Docker image if not needed

[guillaumeeb]

Since https://github.com/dask/dask-jobqueue/pull/545, we have added in most of the CI runs a docker compose build command that was not there (by choice) to use pre-backed Docker images.

Without a docker compose build, as we had updated Python version, CI would fail until the Docker images were built.

But I'm wondering with that addition if we've not made a step back, and now we'll build Docker images every time, even if the Dockerfile has not changed and we could use a pre built one.

For instance, you can see the command here: https://github.com/dask/dask-jobqueue/blob/main/ci/slurm.sh#L10.