[binder] add a Dockerfile with a repo2docker/binder setup

binder/Dockerfile

+FROM dunecommunity/xt-super_debian
+MAINTAINER rene.fritze@wwu.de
+
+# binder wants to set the NB_ vars anyways, so we use it to service both setups
+ARG NB_USER
+ARG NB_UID
+ARG DXT_JUPYTER_TOKEN
+
+USER root
+
+# /home/dune-ci already exists in the base image and needs to be re-used here
+RUN useradd -d /home/dune-ci --shell /bin/bash -u ${NB_UID} -o -c "" -m ${NB_USER} && \
+    chown -R ${NB_USER} /home/dune-ci
+
+ENV JUPYTER_TOKEN=${DXT_JUPYTER_TOKEN} \
+    USER=${NB_USER} \
+    HOME=/home/dune-ci \
+    VENV=${DUNE_BUILD_DIR}/dune-common/dune-env
+
+USER ${NB_USER}
+
+ENV PATH=${VENV}/bin:${PATH}
+
+ENTRYPOINT []
+WORKDIR ${HOME}