With #32406 it will be possible to export the image hash into a file when using docker build. This is great to use in scripts to see both the log outputs and to get the image ID but it is sometime not enough for build scripts.

In linuxkit we would like to tag the resulting image with the hash of the sources used to built it. To do this, we used to hash the sources manually -- now we are taking the Git subtree as a less error-prone approximation. I think it would be a great addition to docker build if that hash of the sources would be computed automatically so it could be used as a tag for fully reproducible builds.

For instance:

docker build --bidfile=build.id .

which would put the hash of the build context into build.id.