You are trying to push a docker image to Nexus Repository 3 and Docker complains with a message similar to:
Error pushing to registry: Put /v2/ubuntu/blobs/uploads/
81ad3e90-544a-4bf6-88bf- acd92b479f69?digest=sha256% 3Aa3ed95caeb02ffe68cdd9fd84406 680ae93d633cb16422d00e8a7c2295 5b46d4: unsupported protocol scheme ""
This is a known issue that occurs when using Docker 1.7.1 (and probably earlier) with Nexus Repository 3.
Upgrade to Docker 1.8.0 or later to solve this, version 1.8.0 is the minimum version supported by Nexus Repository 3.