readme.md

Docker Toolchain

Overview

This section describes how to use the Docker toolchain rules. You should continue reading this README if any of the following apply to you:-

1. You wish to write a new bazel rule that uses docker
2. You wish to extend rules from this repository that are using the docker toolchain. You'll know if your underlying rules is using the docker toolchain but you didn't properly configure it if you see one of the following errors

    In <rule name> rule <build target>, toolchain type
    @io_bazel_rules_docker//toolchains/docker:toolchain_type was requested but only types [] are configured
   

   or

   no matching toolchains found for @io_bazel_rules_docker//toolchains/docker:toolchain_type
   

First read the official Bazel toolchain docs here on how toolchain rules work. After that read How to use the Docker Toolchain

How to use the Docker Toolchain

If you call container_repositories() in your WORKSPACE file, the default docker toolchain is configured. The below explains in more detail what this configuration entails. Depending on your use case you will need replicate this in your repo (e.g., if you are extending these rules) / change this default behavior (e.g., if you need to add more constraints) in different ways.

container_repositories() registers the toolchains exported by this repository:

register_toolchains(
    "@io_bazel_rules_docker//toolchains/docker:default_linux_toolchain",
    "@io_bazel_rules_docker//toolchains/docker:default_windows_toolchain",
    "@io_bazel_rules_docker//toolchains/docker:default_osx_toolchain",
)

These rules by default use the docker binary in your system path. To override this behavior, you need to explicitly call the toolchain configuration function described here.

If you are extending these rules, declare the docker toolchain as a requirement in your rule:

your_rule = rule(
    attrs=...,
    ...
    toolchains=["@io_bazel_rules_docker//toolchains/docker:toolchain_type"],
    implementation=_impl
)

Use the rule as follows in the rule implementation method:

def _impl(ctx):
    # Get the DockerToolchainInfo provider
    toolchain_info = ctx.toolchains["@io_bazel_rules_docker//toolchains/docker:toolchain_type"].info
    # Path to the docker tool
    docker_path = toolchain_info.tool_path
    ...

See toolchain.bzl for the definition of the DockerToolchainInfo provider