swarmbucket: allow overriding config

appengine/cr-buildbucket/doc/swarming.md

 # Buildbucket-Swarming integration, aka Swarmbucket
 
 [go/swarmbucket]
 
 Buildbucket has native integration with swarming and recipes.
 A bucket can be configured so a build for one of configured builders is
 converted to a swarming task that runs a recipe. The results are reported back
 to buildbucket when the task compeltes.
 
 [TOC]
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 A buildbucket build can have `"swarming"` parameter, which is a JSON object with
 optional properties:
 
 * `"recipe"`: specifies a recipe to run, an object with optional properties:
   * `"revision"`: recipe revision
 * `"canary_template"`: specifies whether canary task template must be used:
   * `true`: use the canary template. If not found, respond with an error.
   * `false`: do not use canary template.
   * `null` (default): use canary template with some low probability if it
     exists.
-  
+* `"override_builder_cfg"`: can override builder configuration defined on the
+  server. See also a section about it below.
+
+#### Override configuration dynamically
+
+`swarming.override_builder_cfg` parameter can override builder configuration
+defined on the server. For example, value
+
+
+```javascript
+{
+  "dimensions": ["cores:64"]
+}
+```
+
+(re)defines "cores" dimension to be "64" for this particular build.
+
+The format is defined by the Builder message in
+[project_config.proto](../proto/project_config.proto); in practice, it is JSONPB
+of the message.
+
 ## Tags
 
 A swarming task created by buildbucket has extra tags:
 
 * `buildbucket_hostname:<hostname>`
 * `buildbucket_bucket:<bucket>`
 * `recipe_repository:<repo url>`
 * `recipe_revision:<revision>`
 * `recipe_name:<name>`
 * all tags in `common_swarming_tags` of swarming config.
@@ skipping 23 matching lines @@
 1. Swarming task completes. Swarming sends a message to buildbucket via PubSub.
 1. Buildbucket receives the message and marks the build as completed.
 
 Note: swarming does not notify on task start, so buildbucket marks builds as
 STARTED right after creation.
 
 [go/swarmbucket]: https://goto.google.com/swarmbucket
 [kitchen]: https://chromium.googlesource.com/infra/infra/+/master/go/src/infra/tools/kitchen/
 [isolate_kitchen.py]: https://github.com/luci/recipes-py/blob/master/go/cmd/kitchen/isolate_kitchen.py
 [swarming_task_template.json]: https://chrome-internal.googlesource.com/infradata/config/+/master/configs/cr-buildbucket/swarming_task_template.json