Unset GIT_VERSION in Dockerfile

This is a stop gap solution to the problem that gitaly interprets GIT_VERSION differently from the gitlab-build-images.

While gitaly formats it as vX.Y.Z, in the images derived from gitlab-build-images we set it as X.Y.Z.

These env vars are in fact unrelated and used for different purposes, so in a sense gitaly shadows the image env var, which should be fixed separately.

This fix will merely make sure that the GCK images do not break when gitaly tries to compile git.