DOCKER_CONTENT_TRUST doesn't work with build using BuildKit

Description

It seems docker build is ignoring DOCKER_CONTENT_TRUST when building an image that references one that's not signed.

Steps to reproduce the issue:

1. Set environment variable

export DOCKER_CONTENT_TRUST=1

3. Try to pull unsigned image just to confirm verification is working.

docker pull ericopf/java

4. Create Dockerfile

FROM ericopf/java
ENV TEST 1

5. Build image

docker build -t bad-image . 

6. Confirm image is built

docker image ls bad-iamge

Describe the results you received:

Docker pull fails as expected with the message below:

Error: remote trust data does not exist for docker.io/ericopf/java: notary.docker.io does not have trust data for docker.io/ericopf/java

docker build works fine and image is successfully built from an image that's not signed.

Describe the results you expected:

Failure during docker build showing the same error from docker pull.

Output of docker version:

Client:
 Cloud integration: 1.0.17
 Version:           20.10.7
 API version:       1.41
 Go version:        go1.16.4
 Git commit:        f0df350
 Built:             Wed Jun  2 11:56:22 2021
 OS/Arch:           darwin/amd64
 Context:           default
 Experimental:      true

Server: Docker Engine - Community
 Engine:
  Version:          20.10.7
  API version:      1.41 (minimum version 1.12)
  Go version:       go1.13.15
  Git commit:       b0f5bc3
  Built:            Wed Jun  2 11:54:58 2021
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.4.6
  GitCommit:        d71fcd7d8303cbf684402823e425e9dd2e99285d
 runc:
  Version:          1.0.0-rc95
  GitCommit:        b9ee9c6314599f1b4a7f497e1f1f856fe433d3b7
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

Output of docker info:

docker info    
Client:
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Build with BuildKit (Docker Inc., v0.5.1-docker)
  compose: Docker Compose (Docker Inc., v2.0.0-beta.6)
  scan: Docker Scan (Docker Inc., v0.8.0)

Server:
 Containers: 15
  Running: 1
  Paused: 0
  Stopped: 14
 Images: 7
 Server Version: 20.10.7
 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: io.containerd.runc.v2 io.containerd.runtime.v1.linux runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: d71fcd7d8303cbf684402823e425e9dd2e99285d
 runc version: b9ee9c6314599f1b4a7f497e1f1f856fe433d3b7
 init version: de40ad0
 Security Options:
  seccomp
   Profile: default
 Kernel Version: 5.10.25-linuxkit
 Operating System: Docker Desktop
 OSType: linux
 Architecture: x86_64
 CPUs: 4
 Total Memory: 1.941GiB
 Name: docker-desktop
 ID: EAVA:MGOX:C5VK:TQJU:CYJV:LXOU:5WPD:YGEK:XQD6:IVHM:OMSN:NUYK
 Docker Root Dir: /var/lib/docker
 Debug Mode: true
  File Descriptors: 41
  Goroutines: 46
  System Time: 2021-07-28T11:02:23.326054793Z
  EventsListeners: 4
 HTTP Proxy: http.docker.internal:3128
 HTTPS Proxy: http.docker.internal:3128
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

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

DOCKER_CONTENT_TRUST=1

Start by reproducing the issue with DOCKER_CONTENT_TRUST=1, the unsigned ericopf/java image, and the shown Dockerfile, comparing docker pull with docker build. Trace how buildx handles Dockerfile base-image resolution and trust verification; done means docker build rejects the unsigned base image with the expected trust error.