build: force build stage image to buster

paritytech/ci-linux:production was recently switched from buster to bullseye,

build: force build stage image to buster
paritytech/ci-linux:production was recently switched from buster to bullseye,
54f04b41 · Gilles Filippini · 59111f8f · 54f04b41
Commit 54f04b41 authored 1 year ago by Gilles Filippini
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -38,7 +38,7 @@ check_labels:
    - ./scripts/check_labels.sh $CI_MERGE_REQUEST_LABELS $CI_MERGE_REQUEST_MILESTONE

 .env:
-  image: paritytech/ci-linux:production
+  image: paritytech/ci-linux:1.68.2-buster
  tags:
    - dind