chore: bump MSRV to 1.70, add rust-toolchain (#2580)

This time I've added a `components` section to rust-toolchain.toml file
to explicitly require a rustfmt component. Fingers crossed.

Dockerfile

@@ -1,6 +1,6 @@
 # syntax = docker/dockerfile:1.2
 
-FROM rust:1.65-bullseye as builder
+FROM rust:1.70-bullseye as builder
 WORKDIR app
 COPY . .
 

rust-toolchain.toml

@@ -0,0 +1,3 @@
+[toolchain]
+channel = "1.70"
+components = [ "rustfmt" ]