Fix Dockerfile for Xen integration test

[polgreen]

This fixes the xen integration test. In b96c7ba, the code for versioning was moved from version.h into the Makefile in src/util, and it uses the .git file in the CBMC top directory. This means that the dockerfile now needs access to that .git file.

There are no tests added in this PR because this PR fixes a broken test and makes no changes to CBMC itself.

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