Description
A bug was reported in the moby/moby repository concerning running docker build --quiet --stream . and getting strange output (moby/moby#36812). After doing some investigation the error turned out to be on the docker/cli side. I'll provide the PR for it here.

Steps to reproduce the issue:

1. Run docker build --quiet --stream . in a directory with the following Dockerfile.

FROM alpine:latest

Describe the results you received:

$ docker build --quiet --stream .
&{%!!(MISSING)s(chan error=0xc4200aa2a0) sha256:3fd9065eaf02feaf94d68376da52541925650b81698c53c6824d92ff63f98353
 %!!(MISSING)s(*bytes.Buffer=&{[] 0 [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0] 0}) %!!(MISSING)s(chan struct {}=0xc4200aa300) {%!!(MISSING)s(int32=0) %!!(MISSING)s(uint32=0)}}

Describe the results you expected:
sha256:3fd9065eaf02feaf94d68376da52541925650b81698c53c6824d92ff63f98353

Additional information you deem important (e.g. issue happens only occasionally):
Seems to happen consistently.

output of docker version:
Client:
Version: 18.05.0-ce-rc1
API version: 1.37
Go version: go1.9.5
Git commit: 33f00ce
Built: Thu Apr 26 00:58:56 2018
OS/Arch: darwin/amd64
Experimental: false
Orchestrator: swarm

Server:
Engine:
Version: 18.05.0-ce-rc1
API version: 1.37 (minimum version 1.12)
Go version: go1.10.1
Git commit: 33f00ce
Built: Thu Apr 26 01:06:49 2018
OS/Arch: linux/amd64
Experimental: true

Output of docker info:
Containers: 3
Running: 0
Paused: 0
Stopped: 3
Images: 15
Server Version: 18.05.0-ce-rc1
Storage Driver: overlay2
Backing Filesystem: extfs
Supports d_type: true
Native Overlay Diff: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins:
Volume: local
Network: bridge host ipvlan macvlan null overlay
Log: awslogs fluentd gcplogs gelf journald json-file logentries splunk syslog
Swarm: inactive
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: 773c489c9c1b21a6d78b5c538cd395416ec50f88
runc version: 4fc53a81fb7c994640722ac585fa9ca548971871
init version: 949e6fa
Security Options:
seccomp
Profile: default
Kernel Version: 4.9.87-linuxkit-aufs
Operating System: Docker for Mac
OSType: linux
Architecture: x86_64
CPUs: 2
Total Memory: 1.952GiB
Name: linuxkit-025000000001
ID: SBGZ:YKOB:FW7O:O6IN:Z6TL:HFI3:ESUC:DHAL:XWZV:JBPT:FNMN:RDNT
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): true
File Descriptors: 22
Goroutines: 41
System Time: 2018-05-29T02:19:37.5743496Z
EventsListeners: 2
HTTP Proxy: gateway.docker.internal:3128
HTTPS Proxy: gateway.docker.internal:3129
Registry: https://index.docker.io/v1/
Labels:
Experimental: true
Insecure Registries:
127.0.0.0/8
Live Restore Enabled: false