Fix k8s secret schema (#4819)

pipeline/frontend/yaml/linter/schema/.woodpecker/test-backend-options.yaml

@@ -0,0 +1,10 @@
+steps:
+  - name: Build Container
+    image: woodpeckerci/plugin-kaniko:1.2.1
+    backend_options:
+      kubernetes:
+        secrets:
+          - name: aws-secret
+            key: credentials
+            target:
+              file: /root/.aws/credentials

pipeline/frontend/yaml/linter/schema/schema.json

@@ -762,15 +762,23 @@
       "description": "A target which a native Kubernetes secret maps to.",
       "oneOf": [
         {
-          "env": {
-            "description": "The name of the environment variable which secret maps to.",
-            "type": "string"
+          "type": "object",
+          "additionalProperties": false,
+          "properties": {
+            "env": {
+              "description": "The name of the environment variable which secret maps to.",
+              "type": "string"
+            }
           }
         },
         {
-          "file": {
-            "description": "The filename (path) which secret maps to.",
-            "type": "string"
+          "type": "object",
+          "additionalProperties": false,
+          "properties": {
+            "file": {
+              "description": "The filename (path) which secret maps to.",
+              "type": "string"
+            }
           }
         }
       ]

pipeline/frontend/yaml/linter/schema/schema_test.go

@@ -80,6 +80,10 @@ func TestSchema(t *testing.T) {
 			name:     "Map and Sequence Merge", // https://woodpecker-ci.org/docs/next/usage/advanced-yaml-syntax
 			testFile: ".woodpecker/test-merge-map-and-sequence.yaml",
 		},
+		{
+			name:     "Backend options",
+			testFile: ".woodpecker/test-backend-options.yaml",
+		},
 		{
 			name:     "Broken Config",
 			testFile: ".woodpecker/test-broken.yaml",