```podman-compose config``` cannot print the value of short-form environment variables

[NineSong]

Describe the bug

While short syntax for environment variables has been implemented in #1248, podman-compose config cannot print the value of them.

To Reproduce

Steps to reproduce the behavior:

1. Create a docker-compose.yml file with short-form environment variables.

environment:
  - FOO

2. Create a .env file to assign value to the environment variables.

FOO=bar

3. Run podman-compose config to view the resulting config.

Expected behavior

Print the value

environment:
  FOO: bar

Actual behavior

Print null

environment:
  FOO: null

Environment:

• OS: Linux
• podman version: 5.4.0
• podman compose version: 1.5.0