clear npm cache after installing packages

Author: smlabt

Dockerfile

@@ -25,4 +25,5 @@ RUN apk --no-cache --allow-untrusted -X https://apkproxy.herokuapp.com/sgerrand/
 RUN echo y | android update sdk -a -u -t platform-tools,${ANDROID_APIS},build-tools-${ANDROID_BUILD_TOOLS_VERSION}
 RUN chmod a+x -R $ANDROID_HOME
 RUN chown -R root:root $ANDROID_HOME
-RUN npm i -g --unsafe-perm cordova@${CORDOVA_VERSION} ionic@${IONIC_VERSION}
+RUN npm i -g --unsafe-perm cordova@${CORDOVA_VERSION} ionic@${IONIC_VERSION} && \
+ npm cache clean