Pull docker images when building the Linux VM image (#1882)

I noticed the startup time for Linux VMs are very long due to pulling
docker images
https://github.com/bazelbuild/continuous-integration/blob/ea6e439128fe02244c3537d3895a18c08f055881/buildkite/startup-docker-pdssd.sh#L98

We can pull those images in advance while building the VM image to save
startup time and avoid congestion during CI busy time.