turn `build-image.sh` into a high-level tool about image-tools

[errordeveloper]

Present workflow:

• Update images/compilers with relevant changes
• Merge changes via PR, wait for new image to be built
• Run make update-compilers-image and open another PR
• Merge and wait for new images to be built
• Run ./get-image-digest.sh for each new image, and use output to update Dockerfiles in Cilium

Future workflow:

• Update images/compilers with relevant changes
• Merge changes via PR, wait for new image to be built
• Updates to all dependent images (within this repo) get triggered automatically, manifests stored in manifest.yaml (in master branch of this repo)
• When build tool is used in Cilium or other repos it loads manifest.yaml from this repo
• Build tool handles updates to any dependency images, PR is required to update master and release branches

The manifest.yaml may look like this:

images:
- name: docker.io/cilium/image-compilers
  digest: sha256:2f71fc0c9bdbb9a430ccb8cd86cb1fecc63acff464fc253fe3cd3748488e0e65
  info:
    buildDate: "Tue 26 May 2020 23:47:17 BST"
    source:
      repo: https://github.com/cilium/packaging
      commit: 61d38f67f10928e2a281048aa8f276b51c3c54ff
      dir: https://github.com/cilium/packaging/tree/61d38f67f10928e2a281048aa8f276b51c3c54ff/images/compilers
      dirHash: 818f48aa3eada1d23018f02f3c43a169c51eea1a
    keyPackages:
    - gcc-aarch64-linux-gnu 9.3.0-10ubuntu1
    - gcc 9.3.0-10ubuntu2