Layer improvement for container and develop container

[FedericoMulas]

Started as a fix for the container, ended up as introducing a develop container.

what is changed:

• Added additional unused files to .dockerignore
• Few fix to the container and improved layer stacking. With the change code changes will not trigger the apt installation again but will only copy over the new files.
• Added Docker build example with parameters to docs/docker.md
• Added debug state in the Dockerfile definition. This allow to use the container for development/debugging reusing the main container. It leverage the new wptagent_debug.py
• Partial changes from Fix Markdown in docker.md #625
• Added possibility to pass parameters to the container instead of using environment variables in docs/docker.md. Ideally we don't really need all the additional environment variable with this change, but I did not removed them for backward compatiblity.
• Added examples for the new docker stage into the docs/docker.md

what is new:

• Added .gitattributes to avoid having to fix line endings on sh files on windows.
• Added wptagent_debug.py for starting the agent in debug mode (useful for the debug container).Foundation/wptagent/compare/master...FedericoMulas:wptagent:docker-debug#diff-ae5611658dcd443d7a30575bebcdf15a71b7a0f5abebb18b62d00aa0373cae74) to explain better the changes.

[FedericoMulas]

what is changed:

• Added Docker build example with parameters to docs/docker.md
  what is new:
• Added .gitattributes to avoid having to fix line endings on sh files on windows.