Build demo Docker image failed

```bash
Sending build context to Docker daemon  7.392MB
Step 1/6 : FROM tatsushid/tinycore-python:2.7
 ---> da605b5a5fc5
Step 2/6 : RUN pip install visualdl
 ---> Using cache
 ---> e3501944165c
Step 3/6 : WORKDIR /
 ---> Using cache
 ---> 23abec58902e
Step 4/6 : COPY ./demo/vdl_scratch.py vdl_scratch.py
lstat demo/vdl_scratch.py: no such file or directory
```