fix flake TestRun/Dockerfile_test_copy_symlink

[tejal29]

Fixes #1019

In this PR

1. Replicate docker build behavior by copying the target file to destination link.

2. Create file with same uid and gid as target.

3. Added different scenarios for ln in integration test.

4. Manually verified by building use kaniko and docker.

5. Verified by running the test 10 times

 go test ./integration/...  -run TestRun/test_Dockerfile_test_copy_symlink -count 10 -v --bucket tejal-test --repo gcr.io/tejal-test

...
=== CONT  TestRun/test_Dockerfile_test_copy_symlink
--- PASS: TestRun (0.00s)
    --- PASS: TestRun/test_Dockerfile_test_copy_symlink (0.91s)
        integration_test.go:188: diff = [
              {
                "Image1": "gcr.io/tejal-test/docker-dockerfile_test_copy_symlink",
                "Image2": "gcr.io/tejal-test/kaniko-dockerfile_test_copy_symlink",
                "DiffType": "File",
                "Diff": {
                  "Adds": null,
                  "Dels": null,
                  "Mods": null
                }
              },
              {
                "Image1": "gcr.io/tejal-test/docker-dockerfile_test_copy_symlink",
                "Image2": "gcr.io/tejal-test/kaniko-dockerfile_test_copy_symlink",
                "DiffType": "Metadata",
                "Diff": {
                  "Adds": [],
                  "Dels": []
                }
              }
            ]
PASS
ok  	github.com/GoogleContainerTools/kaniko/integration	140.239s


```

[cvgw]

Why the change from link to copy? I'm struggling to connect this change to a flaky test.

[tejal29]

This test was flaky because of 2 reasons

1. The link value was /tmp/target.
   We were linking kaniko/0/tmp/link to /tmp/target - should have been /kaniko/0/tmp/link
2. The target file /tmp/target was getting wiped out before call to os.Symlink(link, dest) . - this caused flakiness.
   (I still don't know why this is caused. However, imitating docker by actually creating the destlink as a regular file with contents of target file fixed the flakiness)

In the description, i mentioned i fixed the CopySymlink to replicate docker behavior. Docker copies the target file to destination link and hence i replaced os.Symlink to CopyFile.

[cvgw]

Could you explain this code a bit? I'm having trouble wraping my head around it.

It looks like linkPath is the resolved path of src joined with destDir. I'm confused how we can be sure this path exists.

[tejal29]

This was error 1 mentioned in here

This ensures, we are copying/creating the required file in kaniko sratch workspace.