Make application dockerised

Update mail_app to also have different forms of configuration outside of a single base yaml file. Allows local or production configurations to be established.

configuration/base.yaml

@@ -0,0 +1,8 @@
+application:
+  port: 8000
+database:
+  host: "localhost"
+  port: 5432
+  username: "postgres"
+  password: "password"
+  database_name: "newsletter"

configuration/local.yaml

@@ -0,0 +1,2 @@
+application:
+  host: 127.0.0.1

configuration/production.yaml

@@ -0,0 +1,2 @@
+application:
+  host: 0.0.0.0