compose: env_file parsing does not handle unquoted whitespace cleanly

Description

Using an env_file with unescaped whitespace keys breaks compose. Additionally, the error message does not really tell the user which file / line is in issue, or that it’s an env_file issue at all

Steps to reproduce the issue:

docker-compose.yml:

services:

  alpine:
    image: alpine
    env_file: variables.env

variables.env:

VAR=foo bar baz

run docker compose config

Describe the results you received:

% docker compose config
key cannot contain a space

Because the error message does not mention the env file, with a large Yaml config it’s very unclear where the problem may lie

Describe the results you expected:

This works with v1:

% docker-compose config
services:
  alpine:
    environment:
      VAR: foo bar baz
    image: alpine
version: '3.9'

If the whitespace in the env file is valid, compose should behave the same as v1.

If it’s not possible to change the env parsing at this stage, there needs to be an error message that makes it clear what has happened. Ideally we’d point to the line number in the .env file, but if that’s not technically possible we could point to the line number of the env_file: in the Yaml

Output of docker compose version:

Docker Compose version v2.0.0

Output of docker info:

Client:
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Build with BuildKit (Docker Inc., v0.6.3)
  compose: Docker Compose (Docker Inc., v2.0.0)
  scan: Docker Scan (Docker Inc., v0.8.0)

Server:
 Containers: 42
  Running: 16
  Paused: 0
  Stopped: 26
 Images: 73
 Server Version: 20.10.8
 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: e25210fe30a0a703442421b0f60afac609f950a3
 runc version: v1.0.1-0-g4144b63
 init version: de40ad0
 Security Options:
  seccomp
   Profile: default
 Kernel Version: 5.10.47-linuxkit
 Operating System: Docker Desktop
 OSType: linux
 Architecture: aarch64
 CPUs: 4
 Total Memory: 11.68GiB
 Name: docker-desktop
 ID: 552J:DQQK:IROJ:YEMG:H5VB:EUS5:5MEX:VKDI:CQAG:KHXR:CLTY:TJY5
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 HTTP Proxy: http.docker.internal:3128
 HTTPS Proxy: http.docker.internal:3128
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: true
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false