Fix bug in extracting hardlinks

When we execute multistage builds, we store the fs of each intermediate
stage at /kaniko/<stage number> if it's used later in the build. This
created a bug when extracting hardlinks, because we weren't appending
the new directory to the link path.

So, if `/tmp/file1` and `/tmp/file2` were hardlinked, kaniko was trying
to link `/kaniko/0/tmp/file1` to `/tmp/file2` instead of
`/kaniko/0/tmp/file2`. This change will append the correct directory to
the link, and fixes GoogleContainerTools#437 GoogleContainerTools#362 GoogleContainerTools#352 GoogleContainerTools#342.

integration/dockerfiles/Dockerfile_test_hardlink

@@ -1,5 +1,16 @@
+FROM composer@sha256:5c4bd89217b50125f28e08d9f16414ecb75f90ce9b773605472b35cd55f3e5c0 AS composer
+FROM php@sha256:9fe20c8003a12f5907ffdc1d7ec435b4ca4226fa4342b94cec4d66189a439f17
+COPY --from=composer /usr/bin/composer /usr/bin/composer
+
+COPY ./ /app
+WORKDIR /app
+
+RUN ls -l
+
+FROM jboss/base-jdk@sha256:138591422fdab93a5844c13f6cbcc685631b37a16503675e9f340d2503617a41
+
 FROM gcr.io/kaniko-test/hardlink-base:latest
 RUN ls -al /usr/libexec/git-core/git /usr/bin/git /usr/libexec/git-core/git-diff
 RUN stat /usr/bin/git
 RUN stat /usr/libexec/git-core/git
-RUN git --version > /git-version
+RUN git --version > /git-version

pkg/util/fs_util.go

@@ -245,8 +245,8 @@ func extractFile(dest string, hdr *tar.Header, tr io.Reader) error {
 				return errors.Wrapf(err, "error removing %s to make way for new link", hdr.Name)
 			}
 		}
-
-		if err := os.Link(filepath.Clean(filepath.Join("/", hdr.Linkname)), path); err != nil {
+		link := filepath.Join(dest, hdr.Linkname)
+		if err := os.Link(filepath.Clean(filepath.Join("/", link)), path); err != nil {
 			return err
 		}