fix(android): pin docker image to 4.2 for JDK8 (react-native-community#121)

This is needed so that the JDK options work, see:
https://discuss.circleci.com/t/unrecognized-vm-option-usecgroupmemorylimitforheap/37088
Related react-native-community#120

Author: mikehardy

src/executors/linux_android.yml

@@ -12,7 +12,7 @@ parameters:
     type: string
     default: medium
 docker:
-  - image: reactnativecommunity/react-native-android
+  - image: reactnativecommunity/react-native-android:4.2
 resource_class: <<parameters.resource_class>>
 environment:
   - _JAVA_OPTIONS: <<parameters.java_options>>