Bug 1414558 - Build Clang toolchain on a Docker volume; r=froydnj

Before, the build root was not in a Docker cache or volume. With
current Docker works, that meant AUFS. We know AUFS is slow under
I/O load and can cause random failures due to missing data after
writes.

This commit changes the build root to a known Docker volume, which
will be backed by EXT4 and won't have the problems of AUFS.

MozReview-Commit-ID: 6WOH0yednAv

build/build-clang/build-clang.py

@@ -323,9 +323,9 @@ def prune_final_dir_for_clang_tidy(final_dir):
 if __name__ == "__main__":
     # The directories end up in the debug info, so the easy way of getting
     # a reproducible build is to run it in a know absolute directory.
-    # We use a directory in /builds/slave because the mozilla infrastructure
-    # cleans it up automatically.
-    base_dir = "/builds/slave/moz-toolchain"
+    # We use a directory that is registered as a volume in the Docker image.
+    base_dir = "/builds/worker/workspace/moz-toolchain"
+
     if is_windows():
         # TODO: Because Windows taskcluster builds are run with distinct
         # user IDs for each job, we can't store things in some globally