Docker client doesn't respect ALL_PROXY for any command

Description

Trying to issue docker client commands such as docker ps, docker images, docker run, etc. through a proxy to a remote daemon server.

docker ps seems to be respecting HTTP_PROXY env, but none of the commands respect ALL_PROXY settings.

Steps to reproduce the issue:

1. Run a remote docker daemon on port 2376 with TLS enabled on machine A
2. Configure iptables of machine A to only accepts connections on port 2376 from machine B.
3. Setup a socks/https proxy (I'm using shadowsocks) on machine B.
4. Run the command below on machine C to issue a docker command to A through B's proxy.

ALL_PROXY=socks5://127.0.0.1:1080 DOCKER_HOST=tcp://xx.xx.xx.xx:2376 docker images

Describe the results you received:

Cannot connect to the Docker daemon at tcp://xx.xx.xx.xx:2376. Is the docker daemon running?

Describe the results you expected:
List of images on machine A

Additional information:

• docker ps will correctly work as expected with HTTP_PROXY but not ALL_PROXY.
• Many commands like docker images won't work on HTTP_PROXY either.
• After allowing connections from every client to port 2376 on machine A, docker images was working as expected. Meaning it doesn't respect proxy settings.

Output of docker version:

Client:
 Version:	17.12.0-ce
 API version:	1.35
 Go version:	go1.9.2
 Git commit:	c97c6d6
 Built:	Wed Dec 27 20:10:36 2017
 OS/Arch:	linux/amd64
Cannot connect to the Docker daemon at tcp://47.91.72.126:2376. Is the docker daemon running?

# This is the actual version of the server.
Server:
 Engine:
  Version:	17.12.0-ce
  API version:	1.35 (minimum version 1.12)
  Go version:	go1.9.2
  Git commit:	c97c6d6
  Built:	Wed Dec 27 20:12:46 2017
  OS/Arch:	linux/amd64
  Experimental:	false

Output of docker info (without running through proxy):

Containers: 5
 Running: 0
 Paused: 0
 Stopped: 5
Images: 132
Server Version: 17.12.0-ce
Storage Driver: overlay
 Backing Filesystem: extfs
 Supports d_type: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins:
 Volume: local
 Network: bridge host 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: 89623f28b87a6004d4b785663257362d1658a729
runc version: b2567b37d7b75eb4cf325b77297b140ea686ce8f
init version: 949e6fa
Security Options:
 seccomp
  Profile: default
 selinux
Kernel Version: 3.10.0-693.11.6.el7.x86_64
Operating System: CentOS Linux 7 (Core)
OSType: linux
Architecture: x86_64
CPUs: 1
Total Memory: 1.796GiB
Name: xxxxxx.internal
ID: xxxxx
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): false
Registry: https://index.docker.io/v1/
Labels:
Experimental: false
Insecure Registries:
 127.0.0.0/8
Live Restore Enabled: false

Additional environment details:
Docker host is installed on bare metal.

Reproduce the reported command with ALL_PROXY, DOCKER_HOST, and a remote TLS daemon, then compare it with the stated HTTP_PROXY behavior across docker client commands. Trace the Docker CLI's proxy handling for remote daemon connections. Done means commands such as docker images consistently use ALL_PROXY and return the remote daemon's results.