Bug 1419638 - Allow to pass arguments to docker when building docker-images. r=dustin

Ideally, we'd simply use the --build-arg docker argument along with ARG
in the Dockerfile, but that's only supported from Docker API 1.21, and
we're stuck on 1.18 for the moment.

So we add another hack to how we handle the Dockerfile, by adding a
commented syntax that allows to declare arguments to the Dockerfile.

The arguments can be defined in the docker images kind.yml file through
the `args` keyword. Under the hood, they are passed down to the docker
image task through the environment. The mach taskcluster-build-image
command then uses the corresponding values from the environment to
generate a "preprocessed" Dockerfile for its context.

taskcluster/taskgraph/transforms/docker_image.py

@@ -34,6 +34,9 @@ docker_image_schema = Schema({
     # relative path (from config.path) to the file the docker image was defined
     # in.
     Optional('job-from'): basestring,
+
+    # Arguments to use for the Dockerfile.
+    Optional('args'): {basestring: basestring},
 })
 
 
@@ -50,9 +53,11 @@ def fill_template(config, tasks):
     for task in tasks:
         image_name = task.pop('name')
         job_symbol = task.pop('symbol')
+        args = task.pop('args', {})
 
         context_path = os.path.join('taskcluster', 'docker', image_name)
-        context_hash = generate_context_hash(GECKO, context_path, image_name)
+        context_hash = generate_context_hash(
+            GECKO, context_path, image_name, args)
 
         description = 'Build the docker image {} for use by dependent tasks'.format(
             image_name)
@@ -120,6 +125,9 @@ def fill_template(config, tasks):
             },
         }
 
+        for k, v in args.items():
+            taskdesc['worker']['env'][k] = v
+
         add_optimization(
             config, taskdesc,
             cache_type="docker-images.v1",