Switch back from python:3.6-slim-stretch to python:3.6

Turns out slim-stretch doesn't include gcc which means it can't build various
Sanic dependencies. So `datasette publish now ...` was broken.

Fixes #310

datasette/utils.py

@@ -265,7 +265,7 @@ def make_dockerfile(files, metadata_file, extra_options, branch, template_dir, p
         install = ['datasette'] + list(install)
 
     return '''
-FROM python:3.6-slim-stretch
+FROM python:3.6
 COPY . /app
 WORKDIR /app
 {spatialite_extras}