Add environement for flutter image

.drone.yml

@@ -3,6 +3,9 @@ kind: pipeline
 name: build
 type: docker
 
+environment:
+  FLUTTER_IMAGE: jonasfranz/flutter:2.10.1
+
 workspace:
   path: "/app"
 
@@ -17,14 +20,14 @@ clone:
 steps:
   - name: build
     pull: always
-    image: jonasfranz/flutter:2.10.1
+    image: ${FLUTTER_IMAGE}
     commands:
       - flutter packages get
       - flutter build apk
 
   - name: lint
     pull: always
-    image: jonasfranz/flutter:2.10.1
+    image: ${FLUTTER_IMAGE}
     depends_on:
       - build
     commands:
@@ -33,7 +36,7 @@ steps:
 
   - name: test
     pull: always
-    image: jonasfranz/flutter:2.10.1
+    image: ${FLUTTER_IMAGE}
     depends_on:
       - build
     commands:
@@ -44,6 +47,9 @@ kind: pipeline
 name: metrics
 type: docker
 
+environment:
+  FLUTTER_IMAGE: jonasfranz/flutter:2.10.1
+
 depends_on:
   - build
 
@@ -57,7 +63,7 @@ platform:
 steps:
   - name: generate_metrics
     pull: always
-    image: jonasfranz/flutter:2.10.1
+    image: ${FLUTTER_IMAGE}
     commands:
       - flutter pub run dart_code_metrics:metrics analyze lib -r html
   - name: convert_to_latex