make BINDERHUB_VERSION a build arg
so that `build.py` can set the version easily
FROM python:3.6-alpine@sha256:00320c25fa72df54c7ef0181c9e55f59f58b1facb943cb825e4c65fb572ac74b
+ARG BINDERHUB_VERSION
RUN pip install --no-cache-dir \
- https://github.com/jupyterhub/binderhub/archive/dafb7f12b09.zip
+ https://github.com/jupyterhub/binderhub/archive/${BINDERHUB_VERSION}.zip
ADD binderhub_config.py /etc
WORKDIR /etc
