Add smaller docker demo VisualDL server: (#211)

docker build -f Dockerfile.demo -t visualdl:demo .
docker run -p 8040:8040 visualdl:demo
docker run -p 8040:8040 -v <YOUR_LOG_DIR>:/scratch_log visualdl:demo

Dockerfile.demo

+FROM tatsushid/tinycore-python:2.7
+
+RUN pip install visualdl
+WORKDIR /
+COPY ./demo/vdl_scratch.py vdl_scratch.py
+RUN python /vdl_scratch.py
+
+ENTRYPOINT ["visualDL", "--logdir=/scratch_log"]
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