Add multi-staged docker build and flake8

Adds multi-staged docker build that reduces final image size 5x. Adds
flake8 checks during docker image build. Changes submit run response
from render to redirect.

Change-Id: Iadd278b4922ee223517b0ea1ffdf75c4a7dac719
diff --git a/testrail_bot/control/views.py b/testrail_bot/control/views.py
index ee582eb..68738c5 100644
--- a/testrail_bot/control/views.py
+++ b/testrail_bot/control/views.py
@@ -74,9 +74,7 @@
     report.save()
     process_run.delay(run_id, report.id, path)
 
-    return render(
-        request, "control/report.html",
-        {"report": report.path.read().decode("utf-8"), "report_id": report.id})
+    return redirect("single_report", report.id)
 
 
 def show_help(request):