docker_vcs: cache directory for workspace is not preventing duplicate copy

[130s]

E.g. With the following command, i'd expect no temporary context directory would be created as tmp_context_path is defined. But in reality it gets created.

$ ./docker_builder_vcs.py \
  --dockerfile Dockerfile_ros-melodic \
  --entrypoint_exec entry_point.bash \
  --docker_base_img ros:melodic-ros-base \
  --tmp_context_path /tmp/docker_vcs_20220412102739/ \

Open question

• What should the behavior when tmp_context_path is defined. Should the files in the specified path completely ignored? Or check the diff?