generate-targets: explicitly set container image to Debian 11

Otherwise it would use runner default, which might be unpredictable, as
for example GitLab provided public runners are using `ruby:2.5` as
default container image, but true.cz runners are using `debian:10`. So
lets fix it by using Debian 11 image by default.

Signed-off-by: Petr Štetiar <ynezz@true.cz>

.gitlab/ci/base.yml

@@ -28,6 +28,7 @@
 
 
 generate-targets:
+  image: debian:11
   stage: generate_targets
   script:
     - apt-get update && apt-get install -y git make