Refactor: Will not force user to set SECRET_KEY in environ

+ Allow overwrite in local_settings.py (the original implemetation
  will cause an KeyError)

pinry/settings/docker.py

@@ -1,8 +1,14 @@
+import logging
+
 from .base import *
 
 
 # SECURITY WARNING: keep the secret key used in production secret!
-SECRET_KEY = os.environ['SECRET_KEY']
+if 'SECRET_KEY' not in os.environ:
+    logging.warning(
+        "No SECRET_KEY given in environ, please have a check"
+    )
+SECRET_KEY = os.environ.get('SECRET_KEY', None)
 
 # SECURITY WARNING: don't run with debug turned on in production!
 DEBUG = False