fix error in Dockerfile

Dockerfile

@@ -2,7 +2,7 @@ FROM openjdk:8
 WORKDIR /ragdoc-builder
 ADD . /ragdoc-builder/
 RUN chmod +x ./gradlew start-builder.sh && ./gradlew --no-daemon --version
-RUN fatJar && rm -r gradle/ extendj/ src/ build/ gradlew build.gradle settings.gradle
+RUN ./gradlew fatJar && rm -r gradle/ extendj/ src/ build/ gradlew build.gradle settings.gradle
 ENV BOOT_CLASSPATH="/usr/local/openjdk-8/jre/lib/rt.jar"
 VOLUME ["/src"]
 VOLUME ["/data"]