Restore Docker installation on host machine in setup-docker.sh (#2883)

PR #2821 intended to remove Docker-in-Docker installations from worker
container images, but accidentally also removed Docker from
`buildkite/setup-docker.sh`.

`setup-docker.sh` is the startup script for the host GCE Linux VM that
hosts the Buildkite agent and runs CI jobs in Docker containers. Without
Docker installed on the host, the VM image creation pipeline
(`create-linux-vm-image`) [fails and
hangs](https://buildkite.com/bazel-trusted/create-linux-vm-image/builds/48#01a0ceb7-375d-4ab2-a202-1d7dc31658b7)
with _`"/etc/docker/daemon.json: No such file or directory"`_ error.

This PR restores the Docker installation on the host VM.

TODO: We also need to make sure such problems fail and not just hang
there for hours.