app/src/main/res/drawable/join_container_on_post_shape.xml
<?xml version="1.0" encoding="utf-8"?>
<!-- join_container_on_post from your XD file. -->
<shape xmlns:android="http://schemas.android.com/apk/res/android"
android:shape="rectangle" >
<corners android:radius="10dp" />
<stroke android:color="#cfcfcf" android:width="1dp" />
<solid android:color="#FFFFFF" />
</shape>