Ubuntu docker build error

[ZhichengHuang]

When I use the bash docker/build/build-ubuntu.sh, I get the following error:

1. ERROR: Could not open requirements file: [Errno 2] No such file or directory: '/workspace/dev-requirements.txt'
2. CMake Error at cmake/proto2cpp.cmake:3 (message):
   Error: RELATIVE_PROTOBUF_GENERATE_CPP() called without any proto files
   Call Stack (most recent call first):
   cmake/oneflow.cmake:239 (RELATIVE_PROTOBUF_GENERATE_CPP)
   CMakeLists.txt:134 (include)
3. CMake Error at cmake/swig.cmake:3 (message):
   Error: RELATIVE_SWIG_GENERATE_CPP() called without any .i files
   Call Stack (most recent call first):
   cmake/oneflow.cmake:275 (RELATIVE_SWIG_GENERATE_CPP)
   CMakeLists.txt:134 (include)

[jackalcooper]

This PR should fix this. Could you give it a try? #3504