DEV: use proper upstream repo path for ruby-build (#609)

Author: Gabe Pacuilla

image/base/slim.Dockerfile

@@ -81,7 +81,7 @@ RUN mkdir /jemalloc-new && cd /jemalloc-new &&\
       cd / && rm -rf /jemalloc-new
 
 RUN echo 'gem: --no-document' >> /usr/local/etc/gemrc &&\
-    mkdir /src && cd /src && git clone https://github.com/sstephenson/ruby-build.git &&\
+    mkdir /src && git -C /src clone https://github.com/rbenv/ruby-build.git &&\
     cd /src/ruby-build && ./install.sh &&\
     cd / && rm -rf /src/ruby-build && (ruby-build 2.7.5 /usr/local)