Auto-squashing commits in pull-requests

[ilchenkob]

This is not an issue rather suggestion how to improve overall process of contribution into this repo.

CONTRIBUTING.md states:

When submitting a pull request to the docker-images repo, we ask that you squash your commits before we merge.

But GitHub allows to do it automatically during the pull-request merge. There is an option called Squash and merge your pull request commits

Expected behavior

An author of a pull-request can push few commits into PR respectively to code-review comments and all commits will be squashed when PR will be merged.

Actual behavior

An author needs to squash commits manually whenever new commit has been made.