actions: services: cover the case when no container is specified

When the container for running the steps is specificied, it is setup
differently than when it is implicit. This test adds coverage for both
instead of running all examples with an explicitly specified container
image.

actions/example-service/.forgejo/workflows/test.yml

@@ -2,7 +2,23 @@ on: [push]
 
 jobs:
   #
-  # No volume involved
+  # No volume involved & the container is implicit
+  #
+  simple-no-container:
+    runs-on: docker
+    services:
+      pgsql:
+        image: code.forgejo.org/oci/postgres:15
+        env:
+          POSTGRES_DB: test
+          POSTGRES_PASSWORD: postgres
+    steps:
+    - run: |
+       apt-get update -qq
+       apt-get install -y -qq  postgresql-client
+       PGPASSWORD=postgres psql -h pgsql -U postgres -c '\dt' test
+  #
+  # No volume involved & the container is explicit
   #
   simple:
     runs-on: docker