docker: bump Rust to 1.56

This is required since 1.56 is the first stable release that includes
Rust 2021 edition, which caddy-rs uses (and we ought to use too.)

Author: anirudhb

docker/dev.Dockerfile

@@ -1,4 +1,4 @@
-FROM rust:1.54
+FROM rust:1.56
 
 WORKDIR /usr/src/app
 

docker/migrator.Dockerfile

@@ -1,6 +1,6 @@
 # This Docker image performs migrations on the database
 
-FROM rust:1.54
+FROM rust:1.56
 
 WORKDIR /usr/src/app
 

docker/prod.Dockerfile

@@ -1,4 +1,4 @@
-FROM rust:1.54 AS builder
+FROM rust:1.56 AS builder
 
 WORKDIR /usr/src/app