expand service environment_variables before adding to subs_dict

Also modifies an existing integration test to expect an empty string as `docker-compose` warns that
`ZZVAR3` is not set and defaults it to an empty string per the acutal output here.

```yaml
$ docker-compose -f container-compose.load-.env-in-project.yaml config
WARN[0000] The "ZZVAR3" variable is not set. Defaulting to a blank string.
name: project
services:
  app:
    command:
      - /bin/busybox
      - sh
      - -c
      - env | grep ZZ
    environment:
      ZZVAR1: This value is loaded but should be overwritten
      ZZVAR2: This value is loaded from .env in project/ directory
      ZZVAR3: ""
...
```

Signed-off-by: indra <[email protected]>

podman_compose.py

@@ -270,7 +270,6 @@ def rec_subs(value, subs_dict):
             svc_envs = {k: v for k, v in value['environment'].items() if k not in subs_dict}
             # we need to add `svc_envs` to the `subs_dict` so that it can evaluate the
             # service environment that reference to another service environment.
-            subs_dict.update(svc_envs)
             svc_envs = rec_subs(svc_envs, subs_dict)
             subs_dict.update(svc_envs)
 

tests/integration/test_podman_compose_env_file.py

@@ -233,7 +233,7 @@ def test_taking_env_variables_from_env_files_from_different_directories(self):
                 [
                     'ZZVAR1=This value is loaded but should be overwritten\r',
                     'ZZVAR2=This value is loaded from .env in project/ directory\r',
-                    'ZZVAR3=$ZZVAR3\r',
+                    'ZZVAR3=\r',
                     '',
                 ],
             )

tests/unit/test_rec_subs.py

@@ -26,6 +26,16 @@ class TestRecSubs(unittest.TestCase):
             {"environment": {"non_var": "$$v1", "vx": "$non_var"}, "image": "abc:$non_var"},
             {"environment": {"non_var": "$v1", "vx": "$v1"}, "image": "abc:$v1"},
         ),
+        (
+            "service environment with unpopulated ${VARIABLE:-default} format",
+            {"environment": {"v100": "${v100:-low priority}", "actual-v100": "$v100"}},
+            {"environment": {"v100": "low priority", "actual-v100": "low priority"}},
+        ),
+        (
+            "service environment with populated ${VARIABLE:-default} format",
+            {"environment": {"v1": "${v1:-low priority}", "actual-v1": "$v1"}},
+            {"environment": {"v1": "high priority", "actual-v1": "high priority"}},
+        ),
         # list
         (
             "Values in list are substituted",