fix unpacking archives via ADD

[vm86]

Bugfix: unpacking archives with ADD cos it recieves only a full destination path and ignores WORKDIR's directions.

Dockerfile example:

FROM busybox

WORKDIR /app

ADD test_file.tar .
RUN ls /app

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[vm86]

I signed it!

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CLAs look good, thanks!

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[vm86]

@SamyCoenen @priyawadhwa @tejal29

[priyawadhwa]

LGTM, thanks for fixing this!