Build our docker container

As a first step to our build in darklang/dark, we need to build the container, and save it for future steps by pushing it to a repo. However, we do not want to share the keys to push to the docker container publicly (obviously). The solution was making a separate repo and not sharing the keys from it.

Update this repository by running ./clone-from-dark-repo ../dark, where the second argument is the path of your local checkout of the dark repo, which should have the dockerfile you want to build.

Run the script, then commit and push, and the tag(s) will be automatically derived from the current git hash - see tag-version.sh. This tag will be visible in the CircleCI build output.

Then go to the darklang/dark repo's .circleci/config.yaml and update .executors.in-container.docker[0].image with the new tag.