Bug 1430037 - Use sparse checkouts for docker image builds. r=dustin

In many cases, building docker images starts on machines that don't have
a cached checkout, and it often takes forever to get a full clone. It
used to be worsened when 3 jobs could run at the same time because the
worker would start up clean, and 3 jobs would be doing a mercurial clone
at the same time, thrashing I/O, but that part is fortunately fixed.

It is still, however, appreciable not to waste time in the mercurial
clone part of image creation.

--HG--
extra : rebase_source : 8c76bc91e1d5102f68c43e1050d61971fef32e9f

build/sparse-profiles/docker-image

@@ -0,0 +1,16 @@
+%include build/sparse-profiles/mach
+
+[include]
+path:taskcluster/
+
+# Result from `grep -hr %include taskcluster/docker | grep -v " taskcluster/" | sort -u`
+path:build/unix/build-gtk3/build-gtk3.sh
+path:build/valgrind/valgrind-epochs.patch
+path:python/mozbuild/mozbuild/action/tooltool.py
+path:testing/config/tooltool-manifests/linux64/releng.manifest
+path:testing/mozharness/external_tools/robustcheckout.py
+path:tools/lint/eslint/eslint-plugin-mozilla/manifest.tt
+path:tools/lint/eslint/manifest.tt
+path:tools/lint/python/flake8_requirements.txt
+path:tools/lint/tox/tox_requirements.txt
+

taskcluster/docker/image_builder/build-image.sh

@@ -24,6 +24,7 @@ CONTEXT_FILE=/builds/worker/workspace/context.tar
 # Run ./mach taskcluster-build-image with --context-only to build context
 run-task \
   --vcs-checkout "/builds/worker/checkouts/gecko" \
+  --sparse-profile build/sparse-profiles/docker-image \
   -- \
   /builds/worker/checkouts/gecko/mach taskcluster-build-image \
   --context-only "$CONTEXT_FILE" \

taskcluster/taskgraph/transforms/docker_image.py

@@ -168,7 +168,7 @@ def fill_template(config, tasks):
             cache_name = 'imagebuilder-v1'
         else:
             worker['docker-image'] = {'in-tree': 'image_builder'}
-            cache_name = 'imagebuilder-{}'.format(_run_task_suffix())
+            cache_name = 'imagebuilder-sparse-{}'.format(_run_task_suffix())
             # Force images built against the in-tree image builder to
             # have a different digest by adding a fixed string to the
             # hashed data.