Supports file merging docker compose files

Hi,

It would be nice to add functionality to merge docker compose files.
https://docs.docker.com/compose/multiple-compose-files/merge/

# Steps to reproduce
Create docker-compose.yaml
```
version: '3.4'
services:
redis:
image: docker.io/redis:7-alpine
command:
- sh
- -c # this is to evaluate the $REDIS_PASSWORD from the env
- redis-server --appendonly yes --requirepass $$REDIS_PASSWORD ## $$ because of docker-compose
env_file: env/redis.env

```
Create docker-compose.override.yaml
```
version: '3.4'
services:
redis:
restart: always
```

# Expected behavior
```
# redis.container
[Container]
EnvironmentFile=env/redis.env
Exec=sh -c 'redis-server --appendonly yes --requirepass $$REDIS_PASSWORD'
Image=docker.io/redis:7-alpine

[Service]
Restart=always
```

Read the linked Docker Compose merge documentation, then trace the repository's compose-file input path to identify where multiple files could be accepted and combined. Done means the two example files produce the shown Quadlet output, with merge behavior matching the documented rules.

Written by the indexing model from the issue text.