Closes#224
* feat(provider): adds plain registry content property
* docs(provider): new plain config property
* chore: comments why the order of auth parsing should not be changed
* updates the docs due to the changes of #147
* adds documentation for the ssh usage option of #153
* updates docs for usage of the os credentials helpers
* adds pathexpand for config_file in docs
* Fixed layout in the documentation.
* Remove notes that only the public registry is supported.
* Added description for environment variables for docker registry authentication.
