fix(docker): generic git refspec in context

[miki725]

• [x] Followed the steps in the contributor's guide: https://crashoverride.com/docs/other/contributing#filing-the-pull-request
• [x] PR title uses semantic commit messages
• [x] Filled out the template to a useful degree
• [x] Updated CHANGELOG.md if necessary

Issue

fixes https://github.com/crashappsec/chalk/issues/350

Description

Docker supports providing full refspec as the context. For example:

docker build https://github.com/user/repo.git#refs/pull/1/merge

Previously chalk only supported tags and branches:

docker build https://github.com/user/repo.git#<branch>
docker build https://github.com/user/repo.git#<tag>

Now chalk also fetches any other explicit refs passed to it.

Testing

➜ make tests args="test_docker.py::test_git_context --slow -x --logs"

tests:--slow