Context is weird when using several config files

Description

It looks like build.context is resolved from current context rather than from config file for second and past files passed through -f. Such behavior prohibits using those extending (second, third, etc) config files on their own.

Steps to reproduce the issue:

1. cd /tmp
2. git clone https://github.com/SCIF/docker-ignore-fail
3. cd docker-ignore-fail
4. docker-compose -f ./docker-compose.yml -f ./b/docker-compose.yml build

Describe the results you received:

When I cloned this repo in /tmp:

File "/usr/lib/python3/dist-packages/docker/utils/build.py", line 184, in rec_walk
for f in os.listdir(current_dir):
PermissionError: [Errno 13] Permission denied: '/tmp/systemd-private-3…'

When I moved whole repo in /tmp/1/:

Building b
ERROR: Cannot locate specified Dockerfile: b/Dockerfile

Describe the results you expected:

Build both services without issues.

Output of docker version:

Docker version 19.03.6, build 369ce74a3c

Output of docker info:

docker info
Client:
 Debug Mode: false

Server:
 Containers: 16
  Running: 7
  Paused: 0
  Stopped: 9
 Images: 149
 Server Version: 19.03.6
 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 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: 35bd7a5f69c13e1563af8a93431411cd9ecf5021
 runc version: dc9208a3303feef5b3839f4323d9beb36df0a9dd
 init version: fec3683
 Security Options:
  apparmor
  seccomp
   Profile: default
 Kernel Version: 5.4.0-3-amd64
 Operating System: Debian GNU/Linux bullseye/sid
 OSType: linux
 Architecture: x86_64
 CPUs: 8
 Total Memory: 15.59GiB
 Name: scif
 ID: FTBI:3QBW:66NA:PYQI:P5GQ:6SRQ:3XTJ:T3MP:6BSE:JFD4:OS4J:P7L3
 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

WARNING: No swap limit support

** docker-compose --version **

docker-compose version 1.25.0, build unknown

Reproduce the issue with docker-compose.yml and b/docker-compose.yml using the documented docker-compose build command. Trace how build.context is resolved for files passed after the first -f; done means both services build from their config-relative contexts without scanning /tmp or failing to locate b/Dockerfile.