Specify source image tag

It is supposed to prevent deprecation.

If they update to newer packages, this build file will still work.
It will be necessary to be updated manually.

Author: ecklm

Dockerfile

@@ -1,7 +1,7 @@
 # build with `docker build -t dockername:tag .`
 # run with `docker run -p 8888:8888 dockername:tag`
 # open http://localhost:8888 in browser
-FROM jupyter/base-notebook
+FROM jupyter/base-notebook:37af02395694
 
 USER root
 RUN /bin/bash -c "conda create --yes -n py27 python=2.7; source activate py27; conda install --yes notebook ipykernel; ipython kernel install"