Building directly from a checkout will pull in uncommitted files to or
more sneakily files that are git ignored, but not docker ignored.
To avoid this, we'll add an option to build from a git archive of HEAD
instead. Docker doesn't provide a way to build directly from a git
repo, so instead we create a tarball of the current HEAD with git
archive and pipe it into the build command.
When building from a git archive, we'll still display the warning about
uncommitted changes, but we won't add the `_uncommitted_...` suffix to
the container name as they won't be included in the build.
Perhaps this should be the default, but we'll leave that decision for
now.
f3b7569032