Try wrap pipeline in array

.drone.jsonnet

@@ -1,16 +1,23 @@
-local compiler_versions = {
+local compiler_versions = [
-    "commands": []
+    "04", "05", "06", "07", "08", "09"
-};
+];
+local test_compiler_version(version) = [
+    "cd code/compiler/%(version)",
+    "mkdir build && cd build",
+    "cd .."
+];
 
-{
+[
-    "kind": "pipeline",
+    {
-    "type": "docker",
+        "kind": "pipeline",
-    "name": "default",
+        "type": "docker",
-    "steps": [
+        "name": "default",
-        {
+        "steps": [
-            "name": "test-compiler",
+            {
-            "image": "gcc",
+                "name": "test-compiler",
-            "commands": []
+                "image": "gcc",
-        }
+                "commands": std.flatMap(test_compiler_version, compiler_versions)
-    ]
+            }
-}
+        ]
+    }
+]