Fix Symlinks not being copied across stages

[tejal29]

Fixes #915

Description
In case of symlink files across multi-stage files, kaniko would only detect change in the symlink.
Logs suggested, it only saved the symlink and not the target. Due to which user saw a build failure.

INFO[0000] Resolved base name alpine:3.11 to alpine:3.11
INFO[0000] Resolved base name scratch to scratch
INFO[0000] Resolved base name alpine:3.11 to alpine:3.11
....
INFO[0015] Taking snapshot of full filesystem...
INFO[0017] Saving file /usr/lib/libstdc++.so.6 for later use.
....
INFO[0018] COPY --from=0 /usr/lib/libstdc++.so.6 /usr/lib/
error building image: error building stage: failed to execute command: symlink libstdc++.so.6.0.27 /usr/lib/libstdc++.so.6: no such file or directory

In this change,

1. For symlinks, we identify the symlink as well as the target as files to saved

/ # /kaniko/executor -f dockerfiles/Dockerfile_test_copy_symlink --context=dir:///workspace --destination=gcr.io/tejal-test/test --tarPath=image.tar
....
INFO[0004] Taking snapshot of full filesystem...        
INFO[0004] Adding whiteout for /kaniko                  
INFO[0007] Saving file /usr/lib/libstdc++.so.6.0.27 for later use 
INFO[0007] Saving file /usr/lib/libstdc++.so.6 for later use 
INFO[0007] Deleting filesystem...                       
INFO[0007] No base image, nothing to extract            
INFO[0007] Unpacking rootfs as cmd COPY --from=t /usr/lib/libstdc++.so.6 /usr/lib/ requires it. 
INFO[0007] Taking snapshot of full filesystem...        
INFO[0008] COPY --from=t /usr/lib/libstdc++.so.6 /usr/lib/ 
INFO[0008] Taking snapshot of files... 

2. When a symlink is copied across mutliple stages, the target file is also copies.
   Success log above.

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Release Notes

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Fix symlinks files copies across multi-stage dockerfiles.

[samos123]

Some nits and questions

[cvgw]

@tejal29 how similar to #943 is this?

[tejal29]

@tejal29 how similar to #943 is this?

I looked at the PR and looks like for symlinks, it fixes copy by copying the target file when a symlink is copied. The code does not change what file is saved.
For cross stage references, for symlink is referenced, we should save both symlink as well as target.

WDYT?
i will check what docker does when copying a symlink and update this section.

[cvgw]

@tejal29 how similar to #943 is this?

I looked at the PR and looks like for symlinks, it fixes copy by copying the target file when a symlink is copied. The code does not change what file is saved.
For cross stage references, for symlink is referenced, we should save both symlink as well as target.

WDYT?
i will check what docker does when copying a symlink and update this section.

Ya I agree, I think they are addressing two similar but different issues.
This PR: src is a symlink
That PR: dest is a symlink