Load the hosts from the contexts before trying to build.
If there is no context, we'll create one. If there is one but the hosts
don't match we'll re-create.
Where we just have a local context, there won't be any hosts but we
still inspect the builder to check that it exists.
Docker does not respect the .dockerignore file when building from a tar.
Instead by default we'll make a local clone into a tmp directory and
build from there. Subsequent builds will reset the clone to match the
checkout.
Compared to building directly in the repo, we'll have reproducible
builds.
Compared to using a git archive:
1. .dockerignore is respected
2. We'll have faster builds - docker can be smarter about caching the
build context on subsequent builds from a directory
To build from the repo directly, set the build context to "." in the
config.
If there are uncommitted changes, we'll warn about them either being
included or ignored depending on whether we build from the clone.
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.
