refactor: Use multistage Docker build to include only built libraries in Docker image (#717)

* Use a Docker multistage build to copy over only the built Python libraries to the final image
* Use 'docker images' to show built image size at end of CI build

Author: matthewfeickert

.github/workflows/ci.yml

@@ -126,4 +126,5 @@ jobs:
     - uses: actions/checkout@v1.2.0
     - name: Build Docker image
       run: |
-        docker build . --file docker/Dockerfile --tag pyhf:$GITHUB_SHA
+        docker build . --file docker/Dockerfile --tag pyhf/pyhf:$GITHUB_SHA
+        docker images

docker/Dockerfile

@@ -1,9 +1,14 @@
-FROM matthewfeickert/python3-scipy-alpine:scipy-1.2.1
+FROM matthewfeickert/python3-scipy-alpine:scipy-1.2.1 as base
+
+FROM base as builder
 COPY . /code
 RUN cd /code && \
     apk add --no-cache git && \
     rm -rf /var/cache/apk/* && \
     python -m pip install --upgrade --no-cache-dir pip setuptools wheel && \
-    python -m pip install --no-cache-dir -e .[xmlio] && \
+    python -m pip install --no-cache-dir .[xmlio] && \
     python -m pip list
+
+FROM base
+COPY --from=builder /usr/local /usr/local
 ENTRYPOINT ["/usr/local/bin/pyhf"]