Investigate if we can use images of previous builds for cache while building new images

[tkilias]

Background

• In (Add option to) Re-use images from a previous build run #245 was asked if we can use previous builds as cache
• We already support the reuse of unchanged images, however, the images are often very similar, as such it might be interesting, if we can reuse images of previous builds or released images on docker hub as cache.
  • see https://docs.docker.com/engine/reference/commandline/build/#specifying-external-cache-sources

Tasks

• Investigate if we can reuse previous images with --cache-from also from the API
• Investigate if we can create images which can be used as cache
• Investigate how do we find the images of the previous builds, because if we change a build step it gets a new hash and we don't know the hash of the previous build

Potential issues

• We store the build_info in the images which contains the hash of the current image and all previous ones. This makes all images unique and caching might not work
• We use the images hash in the FROM part of the Dockerfiles. This probably allows us only to use an image as cache if has the same base images. This means, the previous build steps need to be unmodified.