Include If else statement in docker file

I want to create docker file with conditional statements. i get parameters from outside ( BUILD_TOOL) . This is the code but i got error while building docker image.

i got this error = dockerfile parse error line 23: unknown instruction: ELSE

RUN if [ "$BUILD_TOOL" = "maven" ] ; then 
    RUN mvn clean install;

#if build tool is gradle
else 
    RUN gradle clean;
fi