spack containerize: allow users to customize the base image (#15028)

This PR reworks a few attributes in the container subsection of
spack.yaml to permit the injection of custom base images when
generating containers with Spack. In more detail, users can still
specify the base operating system and Spack version they want to use:

  spack:
    container:
      images:
        os: ubuntu:18.04
        spack: develop

in which case the generated recipe will use one of the Spack images
built on Docker Hub for the build stage and the base OS image in the
final stage. Alternatively, they can specify explicitly the two
base images:

  spack:
    container:
      images:
        build: spack/ubuntu-bionic:latest
        final: ubuntu:18.04

and it will be up to them to ensure their consistency.

Additional changes:

* This commit adds documentation on the two approaches.
* Users can now specify OS packages to install (e.g. with apt or yum)
  prior to the build (previously this was only available for the
  finalized image).
* Handles to avoid an update of the available system packages have been
  added to the configuration to facilitate the generation of recipes
  permitting deterministic builds.

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