cleanup & optimization

- In Dockerfile switched from ADD to COPY for files.
- Configuration moved from using RUN & sed lines, to "COPY sshd_config".
- Dockerfile made more readable
- One less layer used by Dockerfile, e.i layer optimization.
- Removed pwgen as a dependency and replaced with /dev/urandom, pipes, etc...
- Added .dockerfile
- Added sshd_config
- Improved helper script error messages.