Support multiple base images.

[aaqaishtyaq]

Hey, I have a project setup that builds dev container images similar to Gitpod's workspace images. Currently, I am using ubuntu:20.04 as a base image but want to build some images with ubuntu:22.04. Instead of creating all the images with the newer base image, I want something like:

  - name: pyspark-next
    ref:
      - base-22.04
    chunks:
    - lang-nodejs:16.13.0
    - lang-python
    - lang-java:11