reorganize structure of Dockerfiles

[huckl3b3rry87]

@changyuhsuan can you please restructure your cain/image to include all the Docker related files. Please see virgil for an example. Basically it is a standalone thing and there are no Dockerfiles in the main directory in the repo

[yuxuan83]

Currently, the method I use need to place the Dockerfile at the main directory in the repo, since the COPY command could not deal with relative path (ie: ../../ros). I will find a way to solve this issue and put all the Docker related files in /image/cain/ in order to keep the main directory organized and clean.