Copy of symlinks fails when multistaging build

[remibooo]

Describe the bug
Makisu fails to build when copying symlinks from a stage to another (multistaging build)

To Reproduce

I used the following simple Dockerfile:

FROM ubuntu AS builder
RUN mkdir -p /test/
RUN echo blabla >> /test/file
RUN ln -s /test/file /test/symlink
WORKDIR /home/

FROM ubuntu
COPY --from=builder /test/symlink /test/symlink

and ran it with the master version of makisu and the following command:
docker run -v "$PWD":/job remibooo03/makisu:v0.1.13 sh -c "/makisu/bin/makisu/makisu build --commit=explicit --modifyfs=true -t test_v0.1.13_minimal --dest /job/image_minimal.tar -f /job/Dockerfile_minimal /job 2> /job/log_minimal"

It gave the same error whether the flag commit was explicit or implicit.

Expected behavior
A successful built

Additional context
This is the error message I got (the final lines):

{"level":"info","ts":1580291383.9579847,"msg":"* Step 2/2 (commit) : COPY --from=builder /test/symlink /test/symlink  (bdc8ce21)"}
{"level":"info","ts":1580291383.9580278,"msg":"* Executed COPY --from=builder /test/symlink /test/symlink  (bdc8ce21)","duration":0.000017284}
{"level":"error","ts":1580291384.9725418,"msg":"failed to execute build plan: execute stage: build stage 1: build node: do commit: commit: failed to generate diff layer: write diffs: create layer by copy ops: stat src /makisu-storage/sandbox/sandbox055961992/stages/YnVpbGRlcg==/test/symlink: stat /makisu-storage/sandbox/sandbox055961992/stages/YnVpbGRlcg==/test/symlink: no such file or directory"}

This was solved by addind these two lines before the second stage (FROM ) of the dockerfile, in order to turn the symlinks into hardlinks, but obviously quite dirty...

WORKDIR /
RUN find -type l -exec bash -c 'ln -f "$(readlink -m "$0")" "$0"' {} \;