required validation with multiple env on the same line doesn't work

[AmirAliSobhGol]

Description

if an environment is put after another environment variable on the same line, only the one on the right gets validated in swarm mode.
for example:
BUG: ${A_ENV:?requied},${B_ENV}
doesn't validate A_ENV

however
BUG: ${B_ENV} ,${A_ENV:?requied}
works as expected

Steps to reproduce the issue:

1. docker stack deploy -c docker-compose.yml bug

version: '3.6'
services:
  data:
    image: busybox:latest
    environment:
      BUG: ${A_ENV:?required}${B_ENV}

Describe the results you received:
Creating service bug_data

Describe the results you expected:
invalid interpolation format for services.data.environment.BUG: "required variable A_ENV is missing a value: required". You may need to escape any $ with another $.

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

Output of docker version:

(paste your output here)
Docker version 18.06.0-ce, build 0ffa825

Output of docker info:

(paste your output here)
Containers: 67
 Running: 0
 Paused: 0
 Stopped: 67
Images: 1428
Server Version: 18.06.0-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 macvlan null overlay
 Log: awslogs fluentd gcplogs gelf journald json-file logentries splunk syslog
Swarm: active
 NodeID: swm270rgby5lholx628ihjxji
 Is Manager: true
 ClusterID: ep8prb0kbh7v3s6t5ojrb7nlb
 Managers: 1
 Nodes: 1
 Orchestration:
  Task History Retention Limit: 5
 Raft:
  Snapshot Interval: 10000
  Number of Old Snapshots to Retain: 0
  Heartbeat Tick: 1
  Election Tick: 10
 Dispatcher:
  Heartbeat Period: 5 seconds
 CA Configuration:
  Expiry Duration: 3 months
  Force Rotate: 0
 Autolock Managers: false
 Root Rotation In Progress: false
 Node Address: 192.168.21.66
 Manager Addresses:
  192.168.21.66:2377
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: d64c661f1d51c48782c9cec8fda7604785f93587
runc version: 69663f0bd4b60df09991c08812a60108003fa340
init version: fec3683
Security Options:
 apparmor
 seccomp
  Profile: default
Kernel Version: 4.15.0-30-generic
Operating System: Ubuntu 18.04.1 LTS
OSType: linux
Architecture: x86_64
CPUs: 8
Total Memory: 11.31GiB
Name: bml-ThinkPad-Edge-E531
ID: QJIC:UC5B:ZQQR:MBXA:3B2X:EKQY:5B4M:HE62:MVTR:BF5F:SOYL:CJTS
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): false
No Proxy: localhost,127.0.0.1,registry.bamilo.com
Username: amiralisobhgol
Registry: https://index.docker.io/v1/
Labels:
Experimental: false
Insecure Registries:
 127.0.0.0/8
Registry Mirrors:
 https://registry.docker-cn.com/
Live Restore Enabled: false

WARNING: No swap limit support

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