diff --git a/build-tag-push.py b/build-tag-push.py
index a47591d9..be60ca84 100755
--- a/build-tag-push.py
+++ b/build-tag-push.py
@@ -12,9 +12,6 @@ if not user_name:
     print("export DOCKERHUB_USER=zoe")
     exit(1)
 
-# Execute "docker-compose build" and abort if it fails.
-subprocess.check_call(["docker-compose", "build"])
-
 # Get the name of the current directory.
 project_name = os.path.basename(os.path.realpath("."))
 
@@ -22,27 +19,50 @@ project_name = os.path.basename(os.path.realpath("."))
 # (i.e. number of seconds since January 1st, 1970)
 version = str(int(time.time()))
 
-# Load the services from docker-compose.yml.
-stack = yaml.load(open("docker-compose.yml"))
+input_file = os.environ.get(
+    "DOCKER_COMPOSE_YML", "docker-compose.yml")
+output_file = os.environ.get(
+    "DOCKER_COMPOSE_YML", "docker-compose.yml-{}".format(version))
+
+if input_file == output_file == "docker-compose.yml":
+    print("I will not clobber your docker-compose.yml file.")
+    print("Unset DOCKER_COMPOSE_YML or set it to something else.")
+    exit(1)
+
+print("Input file: {}".format(input_file))
+print("Output file: {}".format(output_file))
+
+# Execute "docker-compose build" and abort if it fails.
+subprocess.check_call(["docker-compose", "-f", input_file, "build"])
+
+# Load the services from the input docker-compose.yml file.
+# TODO: run parallel builds.
+stack = yaml.load(open(input_file))
 
 # Iterate over all services that have a "build" definition.
+# Tag them, and initiate a push in the background.
+push_operations = dict()
 for service_name, service in stack.items():
     if "build" in service:
         compose_image = "{}_{}".format(project_name, service_name)
         hub_image = "{}/{}_{}:{}".format(user_name, project_name, service_name, version)
         # Re-tag the image so that it can be uploaded to the Docker Hub.
         subprocess.check_call(["docker", "tag", compose_image, hub_image])
-        # Execute "docker push" to upload the image.
-        subprocess.check_call(["docker", "push", hub_image])
+        # Spawn "docker push" to upload the image.
+        push_operations[service_name] = subprocess.Popen(["docker", "push", hub_image])
         # Replace the "build" definition by an "image" definition,
         # using the name of the image on the Docker Hub.
         del service["build"]
         service["image"] = hub_image
 
+# Wait for push operations to complete.
+for service_name, popen_object in push_operations.items():
+    print("Waiting for {} push to complete...".format(service_name))
+    popen_object.wait()
+    print("Done.")
+
 # Write the new docker-compose.yml file.
-new_compose_file = "docker-compose.yml-{}".format(version)
-with open(new_compose_file, "w") as f:
+with open(output_file, "w") as f:
     yaml.safe_dump(stack, f)
 
-print("You can now use:")
-print("docker-compose -f {}".format(new_compose_file))
+print("Wrote new compose file: {}".format(output_file))