[librsvg-oci-images] rustc: switch to debian buster from testing

[Jordan Petridis <jpetridis src gnome org>]

commit 70f7bb0d720a9411f6857c28ba38f2e3dc4483be
Author: Jordan Petridis <jpetridis gnome org>
Date:   Thu Oct 11 20:08:54 2018 +0300

    rustc: switch to debian buster from testing

 rustc/rustc_template | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/rustc/rustc_template b/rustc/rustc_template
index f4bda38..f97a52b 100644
--- a/rustc/rustc_template
+++ b/rustc/rustc_template
@@ -1,4 +1,4 @@
-FROM debian:testing-slim
+FROM debian:buster-slim
 
 # Librsvg deps
 RUN apt update -yqq && \