In Dockerfile gen script, CUDNN_VERSION should be obtained from docker image

[GuanLuo]

Description The generation looks for "CUDNN_VERSION" environment variable on host system at first, and later use the version in docker image. CUDNN ships with the docker image so it may differs from the one in host system (or the host system doesn't have CUDNN installed), so I think we should always use the version read from docker image when setting cudnn_home