fix: do not delete flutter's .git

Author: valotas

Dockerfile

@@ -81,8 +81,7 @@ USER root
 
 # Flutter
 RUN cd /opt \
- && git clone https://github.com/flutter/flutter.git -b alpha --depth 1 \
- && rm -rf /opt/flutter/.git
+ && git clone https://github.com/flutter/flutter.git -b alpha --depth 1
 
 ENV PATH=$PATH:/opt/flutter/bin