Layer improvement for container and develop container

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 https://github.com/WPO-Foundation/wptagent/pull/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.

catchpoint / WebPageTest.agent