[libc++] Add a utility script to run the Docker image used by builders

Several contributors have been asking me how to reproduce the CI
environment locally. This is the last step towards making that work
out-of-the-box. Basically, just run `libcxx/utils/ci/run-buildbot-container`
and you're good to go.

Differential Revision: https://reviews.llvm.org/D97782

Author: ldionne

libcxx/utils/ci/Dockerfile

@@ -17,19 +17,10 @@
 # The environment variables in `secrets.env` must be replaced by the actual
 # tokens for this to work. These should obviously never be checked in.
 #
-# To start a shell in the Docker image, use:
-#   $ docker run -it --volume "$(git rev-parse --show-toplevel):/llvm" --workdir "/llvm" $(docker build -q .) bash
-#
-# This will fire up the Docker container and mount the root of the monorepo
-# as /llvm in the container. Be careful, the state in /llvm is shared between
-# the container and the host machine.
-#
-# Finally, a pre-built version of this image is available on DockerHub as
-# ldionne/libcxx-builder. To use the pre-built version of the image, use
-#
-#   $ docker pull ldionne/libcxx-builder
-#   $ docker run -it <options> ldionne/libcxx-builder
+# If you're only looking to run the Docker image locally for debugging a
+# build bot, see the `run-buildbot-container` script located in this directory.
 #
+# A pre-built version of this image is maintained on DockerHub as ldionne/libcxx-builder.
 # To update the image, rebuild it and push it to ldionne/libcxx-builder (which
 # will obviously only work if you have permission to do so).
 #

libcxx/utils/ci/run-buildbot-container

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+
+# This script starts a shell in a container running the libc++ build bot Docker
+# image. That image emulates the environment used by libc++'s Linux builders on
+# BuildKite.
+#
+# Once you're inside the shell, you can run the various build jobs with the
+# `run-buildbot` script.
+#
+# This script must be run from within the LLVM monorepo. Furthermore, the
+# monorepo will be mounted as `/llvm` inside the container. Be careful, the
+# state in `/llvm` is shared between the container and the host machine, which
+# is useful for editing files on the host machine and re-running the build bot
+# in the container.
+
+set -e
+
+MONOREPO_ROOT="$(git rev-parse --show-toplevel)"
+if [[ ! -d "${MONOREPO_ROOT}/libcxx/utils/ci" ]]; then
+    echo "Was unable to find the root of the LLVM monorepo; are you running from within the monorepo?"
+    exit 1
+fi
+docker pull ldionne/libcxx-builder
+docker run -it --volume "${MONOREPO_ROOT}:/llvm" --workdir "/llvm" ldionne/libcxx-builder bash