Try wrap pipeline in array

.drone.jsonnet

@@ -1,7 +1,13 @@
-local compiler_versions = {
-    "commands": []
-};
+local compiler_versions = [
+    "04", "05", "06", "07", "08", "09"
+];
+local test_compiler_version(version) = [
+    "cd code/compiler/%(version)",
+    "mkdir build && cd build",
+    "cd .."
+];
 
+[
     {
         "kind": "pipeline",
         "type": "docker",
@@ -10,7 +16,8 @@ local compiler_versions = {
             {
                 "name": "test-compiler",
                 "image": "gcc",
-            "commands": []
+                "commands": std.flatMap(test_compiler_version, compiler_versions)
             }
         ]
     }
+]