clarify the image is build based on Ubuntu 18.04

Author: pietroalbini

README.md

@@ -1,8 +1,8 @@
 # Build environment for Rust crates
 
 This repository contains the source of the Docker container the Rust project
-uses to build third-party crates. It is based on **Debian 9 Stretch**, and
-contains all the native dependencies used by the Rust crates we know of.
+uses to build third-party crates. It is based on **Ubuntu 18.04**, and contains
+all the native dependencies used by the Rust crates we know of.
 
 ## Adding new dependencies