Implement short syntax for env variables in compose.yml "environment:"

This commit allows compose file to directly use environment variable
values in "environment:" section when variables were set in `.env` file.
This functionality was missing, as docker-compose supports both: short
and variable interpolation syntax forms:
environment:
	- FOO
and
environment:
	- FOO=${FOO}
Relevant docker-compose documentation:
https://docs.docker.com/compose/how-tos/environment-variables/set-environment-variables/
podman-compose is more compatible with docker-compose after this change.

Signed-off-by: Monika Kairaityte <monika@kibit.lt>

podman_compose.py

@@ -1159,7 +1159,14 @@ async def container_to_args(
             podman_args.extend(["-e", e])
     env = norm_as_list(cnt.get("environment", {}))
     for e in env:
-        podman_args.extend(["-e", e])
+        # new environment variable is set
+        if "=" in e:
+            podman_args.extend(["-e", e])
+        else:
+            # environment variable already exists in environment so pass its value
+            if e in compose.environ.keys():
+                podman_args.extend(["-e", f"{e}={compose.environ[e]}"])
+
     tmpfs_ls = cnt.get("tmpfs", [])
     if isinstance(tmpfs_ls, str):
         tmpfs_ls = [tmpfs_ls]