zsh completion breaks when optional arguments are combined (`-i -t` -> `-it`)

---

BUG REPORT INFORMATION

Description

docker zsh completion breaks when optional arguments are combined.
Example, when -i and -t are combined to -it

This problem was reported here: https://github.com/robbyrussell/oh-my-zsh/issues/6710

Steps to reproduce the issue:

1. Make sure you are using zsh and check that the docker autocomplete is enabled.
   Type docker then press tab.
   If it shows you:

image   -- Manage images
images  -- List images

then docker zsh autocomplete is enabled.

2. Type the following:
   docker run -it then press tab
   --> **PROBLEM: **A list of executables on the PATH is displayed instead of possible arguments to docker run

Describe the results you received:

When typing docker run -it and pressing tab, a list of executables on the PATH is displayed.

Describe the results you expected:

When typing docker run -it and pressing tab, I expect to get a list of possible arguments to docker run

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

When not combining options, it works fine. Ex, when typing
docker run -i -t and pressing tab, I get:

02f9d6707661  -- 18.04           in ubuntu
0458a4468cbc  -- xenial-20180123 in ubuntu
3fd9065eaf02  -- latest          in alpine
4ea294712533  -- latest          in nate/dockviz
5b117edd0b76  -- 12.04           in ubuntu

Output of docker version:

$ docker version
Client:
 Version:	18.03.0-ce
 API version:	1.37
 Go version:	go1.9.4
 Git commit:	0520e24
 Built:	Wed Mar 21 23:06:22 2018
 OS/Arch:	darwin/amd64
 Experimental:	false
 Orchestrator:	swarm

Server:
 Engine:
  Version:	18.03.0-ce
  API version:	1.37 (minimum version 1.12)
  Go version:	go1.9.4
  Git commit:	0520e24
  Built:	Wed Mar 21 23:14:32 2018
  OS/Arch:	linux/amd64
  Experimental:	true

I tried using the current lastest version available at https://github.com/docker/cli/blob/ec412bdbb47b0cd84d85a0212c7160cec0ee9f02/contrib/completion/zsh/_docker
(put the content of that file to ~/.oh-my-zsh/plugins/docker/_docker)
with the same result.

Output of docker info:

$ docker info
Containers: 8
 Running: 1
 Paused: 0
 Stopped: 7
Images: 16
Server Version: 18.03.0-ce
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: cfd04396dc68220d1cecbe686a6cc3aa5ce3667c
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: 4
Total Memory: 1.952GiB
Name: linuxkit-025000000001
ID: QOY4:456J:JD4J:E36Y:G6FJ:YAJS:K3HI:J23J:NLV2:OFCV:ZNOY:STQT
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): true
 File Descriptors: 31
 Goroutines: 56
 System Time: 2018-04-10T08:07:26.055313Z
 EventsListeners: 2
HTTP Proxy: docker.for.mac.http.internal:3128
HTTPS Proxy: docker.for.mac.http.internal:3129
Registry: https://index.docker.io/v1/
Labels:
Experimental: true
Insecure Registries:
 127.0.0.0/8
Live Restore Enabled: false

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

Using docker for mac.

Start with contrib/completion/zsh/_docker and reproduce the reported behavior using docker run -it and docker run -i -t in zsh with Docker autocomplete enabled. Compare how the completion script handles combined optional arguments; done means docker run -it offers possible Docker run arguments rather than executables from PATH.