[Regression] Environment variables are ignored during stack deployment

[anokun7]

Description
Environment variables used inside a docker-compose.yml are not getting evaluated. Environment variables allows for dynamic properties to be added to docker-compose.yml files to use them in different environments for example.

Steps to reproduce the issue:

1. Example compose file

ᐅ cat test.yml -p
version: "3.3"

services:
  app:
    image: anoop/counter-demo:${version:-latest}

2. Run docker stack deploy using an environment variable

ᐅ version=onbuild docker stack deploy -c test.yml c1
Creating network c1_default
Creating service c1_app

3. Check the image used.

ᐅ docker stack ps c1
ID                  NAME                IMAGE                       NODE                DESIRED STATE       CURRENT STATE           ERROR               PORTS
yj1d99b0z2ty        c1_app.1            anoop/counter-demo:latest   ip-10-1-1-122.ec2   Running             Running 6 seconds ago

4. The image is anoop/counter-demo:latest

Describe the results you received:
The image is anoop/counter-demo:latest

Describe the results you expected:
The image should have been anoop/counter-demo:onbuild as that was the value of the env variable used at the time the stack was deployed

Additional information you deem important (e.g. issue happens only occasionally):
Seems to happen only on the latest docker client. I am using 18.09.0-ce-beta1.
The same issue does not occur on 17.06.2-ee-16

Output of docker version:

Client: Docker Engine - Community
 Version:           18.09.0-ce-beta1
 API version:       1.30 (downgraded from 1.39)
 Go version:        go1.10.4
 Git commit:        78a6bdb
 Built:             Thu Sep  6 22:41:53 2018
 OS/Arch:           darwin/amd64
 Experimental:      false

Server: Docker Enterprise Edition (EE) 2.0
 Engine:
  Version:          17.06.2-ee-16
  API version:      1.30 (minimum version 1.12)
  Go version:       go1.8.7
  Git commit:       9ef4f0a
  Built:            Thu Jul 26 16:42:19 2018
  OS/Arch:          linux/amd64
  Experimental:     false
 Universal Control Plane:
  Version:          3.0.5
  ApiVersion:       1.30
  Arch:             amd64
  BuildTime:        Thu Aug 30 17:47:03 UTC 2018
  GitCommit:        f588f8a
  GoVersion:        go1.9.4
  MinApiVersion:    1.20
  Os:               linux
 Kubernetes:
  Version:          1.8+
  buildDate:        2018-04-26T16:51:21Z
  compiler:         gc
  gitCommit:        8d637aedf46b9c21dde723e29c645b9f27106fa5
  gitTreeState:     clean
  gitVersion:       v1.8.11-docker-8d637ae
  goVersion:        go1.8.3
  major:            1
  minor:            8+
  platform:         linux/amd64
 Calico:
  Version:          v3.0.8
  cni:              v2.0.6
  kube-controllers: v2.0.5
  node:             v3.0.8