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.

Dockerfile

@@ -0,0 +1,11 @@
+FROM rust:1.56
+
+WORKDIR /app
+
+COPY . .
+ENV SQLX_OFFLINE true
+RUN CARGO BUILD --release
+
+ENV APP_ENVIRONMENT production
+
+ENTRYPOINT ["./target/release/mail_app"]