Don't specify python patch version for pipenv

The base container image is `python:3.7-alpine`, but pipenv was
instructed to install packages for python 3.7.3. While this worked, as
soon as a newer patch version would be released the build would fail.

Thus, now pipenv will use any python 3.7 patch version that is
available.

Change-Id: Ia788e4f60586e215883695df9eaab2ef1dbc637f

container/Dockerfile

diff --git a/container/Dockerfile b/container/Dockerfile
index 31738ea..7a925d0 100644
--- a/container/Dockerfile
+++ b/container/Dockerfile
@@ -10,7 +10,7 @@
 COPY dependencies/ ./
 
 RUN pip3 install --no-cache --upgrade pipenv && \
-    pipenv install --python 3.7.3 --system
+    pipenv install --python 3.7 --system
 
 COPY tools/ ./