https://github.com/bazelbuild/bazel-toolchains is a repository where Google hosts Bazel toolchain configs. These configs are required to configure Bazel to issue commands that will execute inside a Docker container via a remote execution environment.
These toolchain configs include:
Release information of toolchain configs can be found at: https://releases.bazel.build/bazel-toolchains.html.
This repository also hosts the skylark rule used to generate toolchain configs.