Also install build in the image

The standard way to use twine is to use "python3 -m build" so having it is useful.

Dockerfile

@@ -9,5 +9,5 @@
       org.opencontainers.image.created=$BUILD_DATE \
       org.opencontainers.image.title="Twine" \
       org.opencontainers.image.source=$VCS_URL
-RUN pip install --no-cache-dir twine
+RUN pip install --no-cache-dir twine build
 ENTRYPOINT ["twine"]