Reorganization Docker files and builds instructions

[beniz]

PR #706 greatly improved the docker images. However, we need a second pass to clean up / reorganize some elements. See the list below:

• the build dir is often volatile in projects that use cmake. I suggest that it is turned into docker as before the PR as it hosts the Docker build documentation as well.
• the *.Dockerfile should not be at the root of the repository, let's have them into the docker directory above.
• if the build.sh script is generic enough that it can be used as an alternative to the building steps we outline in the documentation, then it could/should go at the root of the repository. If this script eases all builds, then we could re-write the build documentation accordingly in README.md.