README is ahead of release

[chrislovecnm]

The readme lists using

http_archive(
    name = "io_bazel_rules_docker",
    sha256 = "29d109605e0d6f9c892584f07275b8c9260803bf0c6fcb7de2623b2bedc910bd",
    strip_prefix = "rules_docker-0.5.1",
    urls = ["https://github.com/bazelbuild/rules_docker/archive/v0.5.1.tar.gz"],
)

# Call this to override the default docker toolchain configuration. This call
# should be placed BEFORE the call to "container_repositories" below to actually 
# override the default toolchain configuration
load("@io_bazel_rules_docker//toolchains/docker:toolchain.bzl",
    docker_toolchain_configure="toolchain_configure"
)
docker_toolchain_configure(name = "docker_config")

But this is incorrect. Version v0.5.1.tar.gz does not have the toolchain in it yet.

http_archive(
    name = "io_bazel_rules_docker",
    sha256 = "29d109605e0d6f9c892584f07275b8c9260803bf0c6fcb7de2623b2bedc910bd",
    strip_prefix = "rules_docker-0.5.1",
    urls = ["https://github.com/bazelbuild/rules_docker/archive/v0.5.1.tar.gz"],
)

load(
    "@io_bazel_rules_docker//container:container.bzl",
    "container_pull",
    container_repositories = "repositories",
)

container_repositories()

I believe is correct

/cc @nlopezgi

[nlopezgi]

the latest release does not include the toolchain changes, please look at the state of README at the release for instructions for that release (https://github.com/bazelbuild/rules_docker/tree/v0.5.1)