build: Update base Dockerfile (#216)

Update base Python version to 3.7 and pin nbgitpuller version to 1.*

Author: yangchoo

CHANGELOG.rst

@@ -1,5 +1,13 @@
 Changelog
 =========
+
+Unreleased
+----------
+
+Fixed
+-----
+- upgraded base docker image to 3.7 to fix nbgitpuller error
+
 v9.4.2
 ------
 

Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.6-slim-buster
+FROM python:3.7-slim-buster
 
 MAINTAINER Strateos <engineering@strateos.com>
 
@@ -30,7 +30,7 @@ ENV PYTHON_EGG_CACHE /python_eggs
 
 # Install Jupyter, nbgitpuller for separate notebook/environment
 RUN pip install --no-cache-dir notebook==5.* && \
-    pip install nbgitpuller
+    pip install nbgitpuller==1.*
 
 # Install TxPy
 RUN pip install 'transcriptic[jupyter, analysis]'