add more buildkit docs

docs/reference/builder.md

@@ -121,6 +121,28 @@ registries.
 When you're done with your build, you're ready to look into [*Pushing a
 repository to its registry*](https://docs.docker.com/engine/tutorials/dockerrepos/#/contributing-to-docker-hub).
 
+
+## BuildKit
+
+Starting with version 18.09, Docker supports a new backend for executing your
+builds that is provided by the [moby/buildkit](https://github.com/moby/buildkit)
+project. The BuildKit backend provides many benefits compared to the old
+implementation. For example, BuildKit can:
+
+* Detect and skip executing unused build stages
+* Parallelize building independent build stages
+* Incrementally transfer only the changed files in your build context between builds
+* Detect and skip transferring unused files in your build context
+* Use external Dockerfile implementations with many new features
+* Avoid side-effects with rest of the API (intermediate images and containers)
+* Prioritize your build cache for automatic pruning
+
+To use the BuildKit backend, you need to set an environment variable 
+`DOCKER_BUILDKIT=1` on the CLI before invoking `docker build`.
+
+To learn about the experimental Dockerfile syntax available to BuildKit-based
+builds [refer to the documentation in the BuildKit repository](https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/experimental.md).
+
 ## Format
 
 Here is the format of the `Dockerfile`:
@@ -224,10 +246,64 @@ following lines are all treated identically:
 #	  dIrEcTiVe=value
 ```
 
-The following parser directive is supported:
+The following parser directives are supported:
 
+* `syntax`
 * `escape`
 
+## syntax
+
+    # syntax=[remote image reference]
+
+For example:
+
+    # syntax=docker/dockerfile
+    # syntax=docker/dockerfile:1.0
+    # syntax=docker.io/docker/dockerfile:1
+    # syntax=docker/dockerfile:1.0.0-experimental
+    # syntax=example.com/user/repo:tag@sha256:abcdef...
+
+This feature is only enabled if the [BuildKit](#buildkit) backend is used.
+
+The syntax directive defines the location of the Dockerfile builder that is used for
+building the current Dockerfile. The BuildKit backend allows to seamlessly use
+external implementations of builders that are distributed as Docker images and 
+execute inside a container sandbox environment.
+
+Custom Dockerfile implementation allows you to:
+  - Automatically get bugfixes without updating the daemon
+  - Make sure all users are using the same implementation to build your Dockerfile
+  - Use the latest features without updating the daemon
+  - Try out new experimental or third-party features
+
+### Official releases
+
+Docker distributes official versions of the images that can be used for building 
+Dockerfiles under `docker/dockerfile` repository on Docker Hub. There are two 
+channels where new images are released: stable and experimental. 
+
+Stable channel follows semantic versioning. For example:
+
+  - docker/dockerfile:1.0.0 - only allow immutable version 1.0.0
+  - docker/dockerfile:1.0 - allow versions 1.0.*
+  - docker/dockerfile:1 - allow versions 1.*.*
+  - docker/dockerfile:latest - latest release on stable channel
+
+The experimental channel uses incremental versioning with the major and minor
+component from the stable channel on the time of the release. For example:
+
+  - docker/dockerfile:1.0.1-experimental - only allow immutable version 1.0.1-experimental
+  - docker/dockerfile:1.0-experimental - latest experimental releases after 1.0
+  - docker/dockerfile:experimental - latest release on experimental channel
+
+You should choose a channel that best fits your needs. If you only want 
+bugfixes, you should use `docker/dockerfile:1.0`. If you want to benefit from
+experimental features, you should use the experimental channel. If you are using 
+the experimental channel, newer releases may not be backwards compatible, so it
+is recommended to use an immutable full version variant.
+
+For master builds and nightly feature releases refer to the description in [the source repository](https://github.com/moby/buildkit/blob/master/README.md).
+
 ## escape
 
     # escape=\ (backslash)
@@ -1627,6 +1703,38 @@ RUN echo "Hello World"
 When building this Dockerfile, the `HTTP_PROXY` is preserved in the
 `docker history`, and changing its value invalidates the build cache.
 
+### Automatic platform ARGs in the global scope
+
+This feature is only available when using the [BuildKit](#buildkit) backend.
+
+Docker predefines a set of `ARG` variables with information on the platform of
+the node performing the build (build platform) and on the platform of the
+resulting image (target platform). The target platform can be specified with
+the `--platform` flag on `docker build`.
+
+The following `ARG` variables are set automatically:
+
+* `TARGETPLATFORM` - platform of the build result. Eg `linux/amd64`, `linux/arm/v7`, `windows/amd64`.
+* `TARGETOS` - OS component of TARGETPLATFORM
+* `TARGETARCH` - architecture component of TARGETPLATFORM
+* `TARGETVARIANT` - variant component of TARGETPLATFORM
+* `BUILDPLATFORM` - platform of the node performing the build.
+* `BUILDOS` - OS component of BUILDPLATFORM
+* `BUILDARCH` - OS component of BUILDPLATFORM
+* `BUILDVARIANT` - OS component of BUILDPLATFORM
+
+These arguments are defined in the global scope so are not automatically
+available inside build stages or for your `RUN` commands. To expose one of
+these arguments inside the build stage redefine it without value.
+
+For example:
+
+```Dockerfile
+FROM alpine
+ARG TARGETPLATFORM
+RUN echo "I'm building for $TARGETPLATFORM"
+```
+
 ### Impact on build caching
 
 `ARG` variables are not persisted into the built image as `ENV` variables are.
@@ -1935,6 +2043,14 @@ required such as `zsh`, `csh`, `tcsh` and others.
 
 The `SHELL` feature was added in Docker 1.12.
 
+## External implementation features
+
+This feature is only available when using the  [BuildKit](#buildkit) backend.
+
+Docker build supports experimental features like cache mounts, build secrets and
+ssh forwarding that are enabled by using an external implementation of the 
+builder with a syntax directive. To learn about these features, [refer to the documentation in BuildKit repository](https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/experimental.md).
+
 ## Dockerfile examples
 
 Below you can see some examples of Dockerfile syntax. If you're interested in