Shave ~800MB off Docker image by replacing repeated git binaries

Change-Id: I5b1d3739a38eecd48ee00a803e0c1bb119a41e17
Reviewed-on: http://review.couchbase.org/44170
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Chris Hillery <ceej@couchbase.com>