Fix multistage copy use cached layer, when it should not

[duythinht]

Fixes #589, #845 and related multistage dockerfile copy command.

Due to multistage dockerfile bug, copy command should not use cached layer when copy a glob, which is result of previous stage.
My workaround is detected CopyCommand.FilesUsedFromContext does not resolve the context from previous stage. So the the command only caching by hash of cmd.String(), which constant itself. After all I make decision to cache only the copy command, which use the file context instead of every copy command from other stage.

Why is this the best approach?

Most of copy --from, is using layer from previous stage, those contains glob of compiled, compressed, and may be retrieved from cached layer before, so we just need to copy instead of trying fetch cache by itself.

• Includes [unit tests]

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[duythinht]

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[abergmeier]

Can we get this reviewed please?

[rearden-steel]

Please review it, this is a real problem for us.

[cvgw]

@duythinht thanks for the PR! However I think this change would prevent COPY --from commands from ever being cached. I think it would preferable to implement a fix that allows those commands to be correctly cached rather than avoiding caching all together. Apologies if I have misunderstood anything.

As a side note, I have also been exploring a fix for these issues. One possible solution is in #880

[rearden-steel]

May be a good solution will be to check if the layer which the file is copied from was cached or not and use or do not use cache for current layer?

[cvgw]

I believe this issue is fixed and this PR can be closed.