* sort .gitignore and add apt_auth.conf to it
* add helper to setup.sh to create apt_auth.conf (user still needs to enter their own credentials)
* only create apt_auth.conf in make if it doesn't already exist
* add tooling to core dockerfile and makefile to use build secrets
* add ignore for hadolint as mounting syntax is currently not supported so far
* update to latest docker on travis for buildkit support
* documentation
* sort gitignore
* add dockerfile syntax definition for build args
