docker app build --build-arg not passed to docker-compose

Description

I have a dockerfile which takes a build arg (it's an access token). This dockerfile is referenced as the build of a service in docker-compose.yml. running docker-compose build --build-arg PAT=mytoken builds successfully, but running docker app build --build-arg PAT=mytoken . -t mytag fails, the docker build has not recieved that build argument:

...
ARG PAT
RUN test -n "$PAT"
...

Steps to reproduce the issue:

Specify ARG PAT in dockerfile
use dockerfile in service with build: dockerfile:
run docker app build with corresponding --build-arg key=value argument

Describe the results you received: argument is not available during docker build; RUN test -n "$PAT" does fails.

Describe the results you expected: argument is available during docker build; RUN test -n "$PAT" does not fail.

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

Output of docker version:

Client: Docker Engine - Community
 Version:           19.03.8
 API version:       1.40
 Go version:        go1.12.17
 Git commit:        afacb8b
 Built:             Wed Mar 11 01:23:10 2020
 OS/Arch:           windows/amd64
 Experimental:      true

Server: Docker Engine - Community
 Engine:
  Version:          19.03.8
  API version:      1.40 (minimum version 1.12)
  Go version:       go1.12.17
  Git commit:       afacb8b
  Built:            Wed Mar 11 01:29:16 2020
  OS/Arch:          linux/amd64
  Experimental:     true
 containerd:
  Version:          v1.2.13
  GitCommit:        7ad184331fa3e55e52b890ea95e65ba581ae3429
 runc:
  Version:          1.0.0-rc10
  GitCommit:        dc9208a3303feef5b3839f4323d9beb36df0a9dd
 docker-init:
  Version:          0.18.0
  GitCommit:        fec3683

Output of docker app version:

Version:               v0.9.1-beta3
Git commit:            9d2c67f8
OS/Arch:               windows/amd64

Output of docker info:

Client:
 Debug Mode: false
 Plugins:
  app: Docker App (Docker Inc., v0.9.1-beta3)
  buildx: Build with BuildKit (Docker Inc., v0.3.1-tp-docker)
  mutagen: Synchronize files with Docker Desktop (Docker Inc., testing)

Server:
 Containers: 2
  Running: 2
  Paused: 0
  Stopped: 0
 Images: 102
 Server Version: 19.03.8
 Storage Driver: overlay2
  Backing Filesystem: <unknown>
  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 local logentries splunk syslog
 Swarm: inactive
 Runtimes: runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 7ad184331fa3e55e52b890ea95e65ba581ae3429
 runc version: dc9208a3303feef5b3839f4323d9beb36df0a9dd
 init version: fec3683
 Security Options:
  seccomp
   Profile: default
 Kernel Version: 4.19.104-microsoft-standard
 Operating System: Docker Desktop
 OSType: linux
 Architecture: x86_64
 CPUs: 4
 Total Memory: 5.805GiB
 Name: docker-desktop
 ID: MHM2:PV3M:DSEZ:TOMU:VI6W:RXMG:YFQH:6D4Q:QB5E:NBX4:DMQV:UH7G
 Docker Root Dir: /var/lib/docker
 Debug Mode: true
  File Descriptors: 55
  Goroutines: 65
  System Time: 2020-07-15T12:31:11.0904404Z
  EventsListeners: 2
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: true
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false
 Product License: Community Engine

WARNING: bridge-nf-call-iptables is disabled
WARNING: bridge-nf-call-ip6tables is disabled

Additional environment details (AWS, VirtualBox, physical, etc.): Windows Home insider build using WSL 2.0 docker backend

docker / app

docker app build --build-arg not passed to docker-compose #806