Use stable Flutter image as base (flutter#2154)

fkorotkov · amirh · commit d7c909c2bf2e · 2019-10-04T13:55:12.000-07:00
Plus force rebuilding of the image
diff --git a/.ci/Dockerfile b/.ci/Dockerfile
@@ -1,5 +1,4 @@
-
-FROM cirrusci/flutter:latest
+FROM cirrusci/flutter:stable
 
 RUN yes | sdkmanager \
     "platforms;android-27" \
