turn build-image.sh into a high-level tool

[errordeveloper]

Use-cases:

There are many ways of building a Docker image, it would be beneficial to Cilium project to provide consistent and reliable method for building image on release as well as for testing, this method should cater for multi-platform builds as well.

Building an image involves a series of base images, it's desirable to reference the by digest for maximum reproducibility. Digests are hard to manage manually, furthermore - digests are simply not human friendly. It would be very helpful to have a human friendly source for mapping digests to sources in git and general information like the build date and description of key facts about images (e.g. software versions).

Requirements:

• make it usable in Cilium repo
• ensure it serves a human-friendly layer on top of automatic git-based tags and image digests
• make sure it easy to use and very simple to maintain

[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