ci: Use upstream golang image

This saves considerable time, as we do not need to apt-get install golang.

.gitlab-ci.yml

@@ -10,11 +10,9 @@ cache:
     - .go
 
 build:
-  image: debian:buster
+  image: golang:1-buster
   stage: build
   before_script:
-    - echo 'deb http://deb.debian.org/debian buster-backports main' > /etc/apt/sources.list.d/backports.list
-    - apt-get update && apt-get -qq -y -t buster-backports install golang git
     - go version
   script:
     - go build -o $CI_PROJECT_NAME-$GOOS-$GOARCH