use multi-stage build

- python 3.7
- build wheels in first stage, install in second
- run with slim image by default instead of full (~200MB vs ~1GB)

Dockerfile

@@ -1,9 +1,28 @@
-FROM python:3.6.3
+ARG PYTHON_VERSION=3.7
+FROM python:${PYTHON_VERSION}
+
+# build wheels in first image
+ADD . /tmp/src
+RUN mkdir /tmp/wheelhouse \
+ && cd /tmp/wheelhouse \
+ && pip3 wheel --no-cache-dir /tmp/src
+
+# run with slim variant instead of full
+# since we won't need compilers and friends
+FROM python:${PYTHON_VERSION}-slim
+
+# we do need git, though
+RUN apt-get update \
+ && apt-get -y install --no-install-recommends git \
+ && rm -rf /var/lib/apt/lists/*
+
+# install repo2docker
+COPY --from=0 /tmp/wheelhouse /tmp/wheelhouse
+RUN pip3 install --no-cache-dir /tmp/wheelhouse/*.whl
+
+# add git-credential helper
+COPY ./docker/git-credential-env /usr/local/bin/git-credential-env
+RUN git config --system credential.helper env
 
 # Used for testing purpose in ports.py
 EXPOSE 52000
-RUN mkdir /tmp/src
-ADD . /tmp/src
-RUN pip3 install --no-cache-dir /tmp/src
-RUN cp /tmp/src/docker/git-credential-env /usr/local/bin/git-credential-env && \
-    git config --system credential.helper env