Avoid images beeing copied into cookbook dir

There were images copied loose into cookbook directory
instead of cookbook/images

Author: mkuzak

Dockerfile

@@ -4,8 +4,8 @@ RUN mkdir /pandas-cookbook
 WORKDIR /pandas-cookbook
 RUN mkdir ./cookbook ./data ./cookbook/images
 
-COPY cookbook/* ./cookbook/
-COPY cookbook/images/* ./cookbook/images/
+COPY cookbook/*.ipynb ./cookbook/
+COPY cookbook/images/*.png ./cookbook/images/
 COPY data/* ./data/
 
 WORKDIR /pandas-cookbook/cookbook/