Can't search private registry with credential helper enabled

Description

docker search fails when using docker-credential-osxkeychain as credential backend.

Steps to reproduce the issue:

1. Have "credsStore": "osxkeychain" in your ~/.docker/config.json
2. docker login ip:port on a DTR
3. docker search ip:port/alpine on a DTR

Describe the results you received:

Error response from daemon: Unexpected status code 500

But looking at the DTR error, this is due to missing authentication.

Additional information you deem important (e.g. issue happens only occasionally):

When using unsecure credential storage, it works. So DTR seems to be ok.

The DTR certificate is self-signed and I added it to insecure-registries in daemon.json

Output of docker version:

Client:
 Version:      18.05.0-ce
 API version:  1.37
 Go version:   go1.9.5
 Git commit:   f150324
 Built:        Wed May  9 22:12:05 2018
 OS/Arch:      darwin/amd64
 Experimental: false
 Orchestrator: swarm

Server:
 Engine:
  Version:      18.05.0-ce
  API version:  1.37 (minimum version 1.12)
  Go version:   go1.10.1
  Git commit:   f150324
  Built:        Wed May  9 22:20:16 2018
  OS/Arch:      linux/amd64
  Experimental: true

Additional environment details (AWS, VirtualBox, physical, etc.):

Using a DTR

Start by reproducing docker search ip:port/alpine with credsStore set to osxkeychain, after logging in to the DTR and configuring its certificate as described. Trace the Docker CLI's search authentication and credential-helper handling. Done means private-registry search authenticates successfully with the credential helper enabled and no longer returns the 500 error.