[BUG] override file !reset for environment vars only works if both compose and override use colon-style syntax

[Paraphraser]

Description

I've been experimenting with override files and, in particular, the "reset value" feature described at:

• docs.docker.com/reference/compose-file

I'm not sure it's working as intended. Here's a summary of my testing:

Steps To Reproduce

Assume the project directory contains a .env file with the line:

TZ=Australia/Sydney

I'm not using TZ for any particular reason other than the service definitions I was working with already had TZ defined such that it would search .env then apply Etc/UTC as a failsafe.

---

Four tests:

1. Compose file for a service contains hyphen-style syntax:

   environment:
     - TZ=${TZ:-Etc/UTC}

   Override file contains matching hyphen-style syntax:

   environment:
     - TZ=!reset null

   Result from running docker config on the service:

   environment:
     TZ: '!reset null'

   and that value gets passed into the container (useless as a TZ).

2. Compose file for a service contains hyphen-style syntax:

   environment:
     - TZ=${TZ:-Etc/UTC}

   Override file contains non-matching colon-style syntax:

   environment:
     TZ: !reset null

   Result from running docker config on the service:

   environment:
     TZ: Australia/Sydney

   and that value gets passed into the container (valid as a TZ but not what was wanted).

3. Compose file for a service contains colon-style syntax:

   environment:
     TZ: ${TZ:-Etc/UTC}

   Override file contains non-matching hyphen-style syntax:

   environment:
     - TZ=!reset null

   Result from running docker config on the service:

   environment:
     TZ: '!reset null'

   and that value gets passed into the container (useless as a TZ).

4. Compose file for a service contains colon-style syntax:

   environment:
     TZ: ${TZ:-Etc/UTC}

   Override file contains matching colon-style syntax:

   environment:
     TZ: !reset null

   Result from running docker config on the service is no environment section. The container does not get TZ passed into the container. This is the expected result of all four tests.

Compose Version

$ docker compose version
Docker Compose version v5.0.2
$ docker-compose version
Docker Compose version v5.0.2

Docker Environment

Client: Docker Engine - Community
 Version:    29.2.1
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.31.1
    Path:     /usr/libexec/docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v5.0.2
    Path:     /usr/libexec/docker/cli-plugins/docker-compose

Server:
 Containers: 6
  Running: 6
  Paused: 0
  Stopped: 0
 Images: 6
 Server Version: 29.2.1
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Using metacopy: false
  Native Overlay Diff: true
  userxattr: false
 Logging Driver: local
 Cgroup Driver: systemd
 Cgroup Version: 2
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local splunk syslog
 CDI spec directories:
  /etc/cdi
  /var/run/cdi
 Swarm: inactive
 Runtimes: io.containerd.runc.v2 runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: dea7da592f5d1d2b7755e3a161be07f43fad8f75
 runc version: v1.3.4-0-gd6d73eb8
 init version: de40ad0
 Security Options:
  seccomp
   Profile: builtin
  cgroupns
 Kernel Version: 6.1.21-v8+
 Operating System: Debian GNU/Linux 12 (bookworm)
 OSType: linux
 Architecture: aarch64
 CPUs: 4
 Total Memory: 3.704GiB
 Name: sec-dev
 ID: f6406dbb-2fc1-4834-9606-5e826871aec6
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
  ::1/128
 Live Restore Enabled: false
 Firewall Backend: iptables

Anything else?

The FOO: example in the documentation uses colon-style syntax and doesn't explicitly rule out hyphen-style syntax. There are a lot of service definitions out there in the big wide world that use hyphen-style syntax so they should probably be supported, irrespective of whether they use matching or mismatching syntax between the compose and override files.

Test 3 is quite intriguing because the value of Australia/Sydney which can only have come from the .env file looks like it is somehow bypassing the override.

I have not experiments with !override or ![reset] forms, nor explored what happens with other hyphen-style clauses (eg depends_on:).

This may be related to #13581 and #11816.