force debian builds to buster

.gitlab-ci.yml

@@ -36,7 +36,7 @@ lint:
 
 .build-deb:
   stage: build
-  image: debian:latest
+  image: debian:buster
   script:
     - echo deb http://deb.debian.org/debian buster-backports main > /etc/apt/sources.list.d/backports.list
     - apt-get update
@@ -128,7 +128,7 @@ build:aarch64:deb:
 
 build:armhf:deb:
   extends: .build-deb
-  image: arm32v7/debian:latest
+  image: arm32v7/debian:buster
   tags: [arm-builder]
   variables:
     ARCH: armhf