Use latest Rust in backend Docker image.

CvX · CvX · commit 520f4e62bd7c · 2018-04-07T14:22:19.000+02:00
The `pest` crate (a dependency of `comrak`) requires at least Rust 1.23 (https://github.com/pest-parser/pest#usage). Currently we test crates.io on Travis using latest stable Rust (and newer). I don't think there's any reason to pin the Docker image to a specific (and outdated) Rust version.
diff --git a/backend.Dockerfile b/backend.Dockerfile
@@ -1,4 +1,4 @@
-FROM rust:1.20-stretch
+FROM rust:latest
 
 RUN apt-get update \
     && apt-get install -y postgresql cmake \

Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,4 @@`
`1`		`-FROM rust:1.20-stretch`
	`1`	`+FROM rust:latest`
`2`	`2`
`3`	`3`	`RUN apt-get update \`
`4`	`4`	`&& apt-get install -y postgresql cmake \`