Bug 1290531 - Add mach taskcluster-build-image command; r=dustin

Docker image building will soon need to use Python in order to produce
the image build context archive.

As the first step towards this, we introduce a Python function that
calls out to build.sh. We also implement a mach command that calls it
so we can test the functionality.

I'm not keen about introducing a new mach command. I'd prefer to have
a sub-command instead. I'm not sure what all uses
`mach taskcluster-load-image`. Perhaps we could make a `taskcluster`
top-level command. Or perhaps we could fold these image commands into
`mach taskgraph`? Either way, the mach side of this isn't terribly
important to the commit series: most of the code will live inside a
Python module outside of mach.

MozReview-Commit-ID: AI8p6H4psNh

taskcluster/mach_commands.py

@@ -228,7 +228,7 @@ class MachCommands(MachCommandBase):
 
 
 @CommandProvider
-class LoadImage(object):
+class TaskClusterImagesProvider(object):
     @Command('taskcluster-load-image', category="ci",
              description="Load a pre-built Docker image")
     @CommandArgument('--task-id',
@@ -253,3 +253,16 @@ class LoadImage(object):
         except Exception:
             traceback.print_exc()
             sys.exit(1)
+
+    @Command('taskcluster-build-image', category='ci',
+             description='Build a Docker image')
+    @CommandArgument('image_name',
+                     help='Name of the image to build')
+    def build_image(self, image_name):
+        from taskgraph.docker import build_image
+
+        try:
+            build_image(image_name)
+        except Exception:
+            traceback.print_exc()
+            sys.exit(1)

taskcluster/taskgraph/docker.py

@@ -15,6 +15,7 @@ import urllib2
 from taskgraph.util import docker
 
 GECKO = os.path.realpath(os.path.join(__file__, '..', '..', '..'))
+IMAGE_DIR = os.path.join(GECKO, 'testing', 'docker')
 INDEX_URL = 'https://index.taskcluster.net/v1/task/docker.images.v1.{}.{}.hash.{}'
 ARTIFACT_URL = 'https://queue.taskcluster.net/v1/task/{}/artifacts/{}'
 
@@ -61,3 +62,14 @@ def load_image_by_task_id(task_id):
 
     print("The requested docker image is now available as", name)
     print("Try: docker run -ti --rm {} bash".format(name))
+
+
+def build_image(name):
+    """Build a Docker image of specified name.
+
+    Output from image building process will be printed to stdout.
+    """
+    args = [os.path.join(IMAGE_DIR, 'build.sh'), name]
+    res = subprocess.call(args, cwd=IMAGE_DIR)
+    if res:
+        raise Exception('error building image')