compose: docker compose does not use current buildkit builder

Description

docker compose build does not use the current buildkit builder. both docker build and docker-compose build use this builder, which I need because it contains cache mount data needed for a particularly slow npm ci command.

Steps to reproduce the issue:

1. docker buildx create --name cacheable --use
2. docker buildx inspect --bootstrap
3. docker compose build <service_name>

Describe the results you received: The build runs and completes successfully, but does not use my currently selected builder. This can be identified by (a) the slow build and (b) if the builder is stopped beforehand with docker stop buildx_buildkit_cacheable0 then it is still stopped after the build completes.

Describe the results you expected: I expect docker compose build to use my current buildkit builder, named buildx_buildkit_cacheable0

Additional information you deem important (e.g. issue happens only occasionally): Both docker build and docker-compose build will start the currently selected builder container (if stopped) and use it to build, which leads me to believe this might be a bug in docker compose.

Environment contains these vars: COMPOSE_DOCKER_CLI_BUILD=1 DOCKER_BUILDKIT=1

I have tried setting BUILDKIT_HOST=docker-container://buildx_buildkit_cacheable0 without any change in the behaviour of docker compose build.

Output of docker version:

Client: Docker Engine - Community
 Cloud integration: 1.0.17
 Version:           20.10.6
 API version:       1.41
 Go version:        go1.13.15
 Git commit:        370c289
 Built:             Fri Apr  9 22:46:38 2021
 OS/Arch:           linux/amd64
 Context:           default
 Experimental:      true

Server: Docker Engine - Community
 Engine:
  Version:          20.10.6
  API version:      1.41 (minimum version 1.12)
  Go version:       go1.13.15
  Git commit:       8728dd2
  Built:            Fri Apr  9 22:44:49 2021
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.4.4
  GitCommit:        05f951a3781f4f2c1911b05e61c160e9c30eaa8e
 runc:
  Version:          1.0.0-rc93
  GitCommit:        12644e614e25b05da6fd08a38ffa0cfe1903fdec
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

Output of docker context show:
You can also run docker context inspect context-name to give us more details but don’t forget to remove sensitive content.

default

Output of docker info:

Client:
 Context:    default
 Debug Mode: false
 Plugins:
  app: Docker App (Docker Inc., v0.9.1-beta3)
  buildx: Build with BuildKit (Docker Inc., v0.5.1-docker)
  compose: Docker Compose (Docker Inc., 2.0.0-beta.3)
  scan: Docker Scan (Docker Inc., v0.7.0)

Server:
 Containers: 1
  Running: 0
  Paused: 0
  Stopped: 1
 Images: 2
 Server Version: 20.10.6
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: true
  userxattr: false
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Cgroup Version: 1
 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 io.containerd.runc.v2 io.containerd.runtime.v1.linux
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 05f951a3781f4f2c1911b05e61c160e9c30eaa8e
 runc version: 12644e614e25b05da6fd08a38ffa0cfe1903fdec
 init version: de40ad0
 Security Options:
  apparmor
  seccomp
   Profile: default
 Kernel Version: 5.11.0-22-generic
 Operating System: Ubuntu 21.04
 OSType: linux
 Architecture: x86_64
 CPUs: 8
 Total Memory: 31.24GiB
 Name: W540
 ID: QNQR:7RST:WACE:AHDH:E6OR:II4B:QFD6:B7LR:K3I3:4VDE:3VPD:ZZI3
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Username: strophy
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

Additional environment details (AWS ECS, Azure ACI, local, etc.): local