changed hardcoded username with variable one (keras-team#9677)

* changed hardcoded username with variable one

* added variable username for installation of python packages

Author: stamate

docker/Dockerfile

@@ -29,11 +29,11 @@ ENV NB_USER keras
 ENV NB_UID 1000
 
 RUN useradd -m -s /bin/bash -N -u $NB_UID $NB_USER && \
-    chown keras $CONDA_DIR -R && \
+    chown $NB_USER $CONDA_DIR -R && \
     mkdir -p /src && \
-    chown keras /src
+    chown $NB_USER /src
 
-USER keras
+USER $NB_USER
 
 ARG python_version=3.6