Also install lib32stdc++6 for 32bit

Reference: flutter/flutter#6207

Author: martinlowinski

Dockerfile

@@ -20,7 +20,7 @@ RUN apt-get install -y openjdk-8-jdk wget expect git curl unzip software-propert
 # if we don't specify this, the libstdc++6 we get is the wrong version
 RUN add-apt-repository ppa:ubuntu-toolchain-r/test -y \
  && apt-get update -qq \
- && apt-get install -y libstdc++6 fonts-droid-fallback
+ && apt-get install -y libstdc++6 lib32stdc++6 fonts-droid-fallback
 
 # Gradle
 RUN add-apt-repository ppa:cwchien/gradle -y \