Dockerfiles: Constrain make parallelism to finite value

Change the make parallelism setting (-j) used during docker building
from an infinite number to 8 jobs in parallel. The inifinite
parallelism was causing out-of-memory situations on more modest build
hosts.

Change-Id: I785ad34dd984c010290e76401c4cbb7ddfd63687
Reviewed-on: http://review.couchbase.org/44462
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Dave Rigby <daver@couchbase.com>