FR: Symlink blobs from base image instead of copying over.

[thesayyn]

Currently when an oci_image depends on another one, all the blobs from the base oci_image is copied over. This was done for two reasons;

• Avoid cascading builds.
• Bazel did not allow symlinks inside treeartifacts that pointed outside of the treeartifact.

With bazelbuild/bazel#21263 fixed, now we can have symlinks inside treeartifacts pointing outside.

We expect this to significantly improve build time of oci_image.

[thesayyn]

This will close: #365, #439

[sgammelmark]

It would be great, if the provided tar-files could be symlinked as well.

I have an aspect the creates individual layers for each external python dependency. Incrementally updating the individual layers on a code change is very fast (less than a second), but updating the image can be time consuming for large images (more than 60 s). In my case, the individual blobs are exactly the same file as the tar-file, so a symlink would be far more efficient than copying the layers.

A missing part of this is maybe that the digest of each layer needs to be computed as well? Having a rule or another mechanism for pre-computing that layer digest so layers can be re-used in different images would be great.

Not sure if it fits this issue exactly, but the symlinking part seems to be relevant.

[thesayyn]

It would be great, if the provided tar-files could be symlinked as well.

We have separate tracking issue for that; #508

[thesayyn]

fixed by #560