Secret does not get mounted in Dockerfile at build time

[3rd3]

Description

I am trying to mount a file as a secret, however I am unable to access it.

Steps to reproduce the issue:

Dockerfile:

FROM alpine
RUN --mount=type=secret,id=mysecret,dst=/test cat /test 

docker-compose.yml:

services:
  test:
    build: .
    secrets:
      - mysecret
secrets:
  mysecret:
    file: mysecret

To run:

echo "ok" > mysecret
DOCKER_BUILDKIT=1 docker-compose up -d

Describe the results you received:

The build process prints an error: /test: No such a file or directory

Describe the results you expected:

The build process should print ok.

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

Output of docker compose version:

docker-compose version 1.29.2, build 5becea4c
docker-py version: 5.0.0
CPython version: 3.7.10
OpenSSL version: OpenSSL 1.1.0l  10 Sep 2019

Output of docker info:

Client:                                                                              
 Context:    default                                                                 
 Debug Mode: false                                                                   
                                                                                     
Server:                                                                              
 Containers: 25                                                                      
  Running: 0                                                                         
  Paused: 0                                                                          
  Stopped: 25                                                                        
 Images: 36                                                                          
 Server Version: 20.10.11                                                            
 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: 7b11cfaabd73bb80907dd23182b9347b4245eb5d                        
 runc version: v1.0.3-0-gf46b6ba                                                     
 init version: de40ad0                                                               
 Security Options:                                                                   
  apparmor                                                                           
  seccomp                                                                            
   Profile: default                                                                  
 Kernel Version: 5.10.83-5.ph4-esx                                                   
 Operating System: VMware Photon OS/Linux                                            
 OSType: linux                                                                       
 Architecture: x86_64                                                                
 CPUs: 2                                                                             
 Total Memory: 15.65GiB                                                              
 Name: photon-vm                                                                     
 ID: NY4K:3ABX:F3R6:KZIJ:DBYU:PRJF:3YPL:5VST:MPOV:YOPK:PSB7:TUYV                     
 Docker Root Dir: /var/lib/docker                                                    
 Debug Mode: false                                                                   
 Registry: https://index.docker.io/v1/                                               
 Labels:                                                                             
 Experimental: false                                                                 
 Insecure Registries:                                                                
  127.0.0.0/8                                                                        
 Live Restore Enabled: false                                                         
 Product License: Community Engine                                                   
 Default Address Pools:                                                              
   Base: 10.90.0.0/16, Size: 24                                                      

[3rd3]

Duplicate of #6358.