Update comments in Dockerfile

Author: staticfloat

docker_images/v0.6/Dockerfile

@@ -9,5 +9,10 @@ ENV BINARYBUILDER_DOWNLOADS_CACHE /shards
 ENV BINARYBUILDER_AUTOMATIC_APPLE true
 ENV BINARYBUILDER_USE_CCACHE true
 
+# we'll make this even though the user should mount something in here
+RUN mkdir -p /shards
+
 # Install BinaryBuilder
 RUN julia -e 'Pkg.clone("https://github.com/JuliaPackaging/BinaryBuilder.jl.git"); using BinaryBuilder'
+
+# The user should mount something in /root/.ccache and /shards