clarkb: I have add cache-devstack element, but I found every time build diskimage downloads cirrors from the network
```
diskimages:
  - name: ubuntu-focal
    pause: false
    elements:
      - ubuntu-minal
      - vm
      - simple-init
      - nodepool-base
      - cache-devstack
      - initialize-urandom
      - growroot
      - infra-package-needs
    release: focal
    username: zuul
    env-vars:
      TMPDIR: /opt/dib_tmp
      DIB_CHECKSUM: '1'
      DIB_IMAGE_CACHE: /opt/dib_cache
      DIB_APT_LOCAL_CACHE: '0'
      DIB_DISABLE_APT_CLEANUP: '1'
      DIB_DEBOOTSTRAP_EXTRA_ARGS: '--no-check-gpg'
      DIB_DEBIAN_COMPONENTS: 'main,universe'
      DIB_CUSTOM_PROJECTS_LIST_URL: 'file:///etc/nodepool/repos.yaml'
      GIT_HTTP_LOW_SPEED_TIME: '300'
      GIT_HTTP_LOW_SPEED_LIMIT: '1000'
      ZUUL_USER_SSH_PUBLIC_KEY: '/var/lib/nodepool/.ssh/id_rsa.pub'
```