add ubuntu cpu dockerfile, test=doc

docker/ubuntu18-cpu/Dockerfile

+FROM registry.baidubce.com/paddlepaddle/paddle:2.2.2
+LABEL maintainer="paddlesl@baidu.com"
+
+RUN git clone https://github.com/PaddlePaddle/PaddleSpeech.git /home/PaddleSpeech 
+RUN pip3 uninstall mccabe -y; exit 0;
+RUN pip3 install multiprocess==0.70.12
+RUN pip3 install importlib-metadata==4.2.0
+
+RUN cd /home/PaddleSpeech/audio
+RUN python setup.py bdist_wheel
+RUN pip install dist/*.whl
+
+RUN cd /home/PaddleSpeech
+RUN python setup.py bdist_wheel
+RUN pip install dist/*.whl
+
+WORKDIR /home/PaddleSpeech/