This PR adds experimental support for using moby/buildkit as a backend for docker build. #32925 #34227

The support is only enabled in the experimental daemon, and user needs to opt-in in docker/cli to start using it.

# in daemon
dockerd -D --experimental

# in Docker CLI
export DOCKER_BUILDKIT=1
docker build .

Docker CLI branch with BuildKit support enabled is in https://github.com/tonistiigi/docker-cli/tree/experimental-buildkit

In the API, version field allows switching between different build backends.

This enables lots of new capabilities, including parallel build steps and requests, skipping unused stages, efficient incremental builds, build context file detection, remote cache support, graceful cancellation, build cache issues, etc. It also enables using experimental Dockerfile features without requiring to update Moby by using the BuildKit frontend images.

The technical side of this integration is a temporary (and somewhat limited) solution as BuildKit is based on containerd storage and that has not been integrated into Moby yet. The current solution provides adapters to some components to provide compatibility between things like containerd snapshots and Moby layerstore/storage drivers or image store. When containerd storage support lands in Moby, all these adapters should be removed entirely, and BuildKit can use containerd directly. This temporary solution allows us to get feedback before we feel comfortable to move it out of experimental.

The tests will not currently use BuildKit in the PR as it requires opt-in and a new CLI binary. We have run the tests manually. Some of them need to be reworked for output change or a side-effect removal. We will continue addressing some of the test results that we are still investigating.

Known issues (some of them will not make it into 18.06 release):

• no automatic GC (manual system df and system prune work)
• pulling schema1 images during build
• automatic path detection in dockerfile doesn't detect unused symlinks
• resource limitation and --net options: execution completely bypasses the daemon, net is host.
• windows support (depending on containerd runtime integration)
• --pull (currently existing images in image store always take precedence)
• USER doesn't set additional groups
• --iidfile is not enabled in CLI
• No way to export remote build cache. Build cache exported with standalone BuildKit can be imported with --cache-from.

@tiborvass @AkihiroSuda

depends on #36895