.gitlab-ci.yml: update debian:buster base image

16 jobs for master in 63 minutes and 31 seconds (queued for 1 second)
Status Job ID Name Coverage
  Stage0
passed #4026
pull base distributions

00:00:27

 
  Stage1
passed #4031
base:10

00:06:16

passed #4032
base:16.04

00:05:28

passed #4033
base:16.10

00:05:37

passed #4029
base:8

00:05:06

passed #4030
base:9

00:05:11

passed #4027
docker-builder

00:00:37

passed #4028
proxy

00:00:40

 
  Stage2
passed #4034
base:8-backports

00:01:04

passed #4035
dune:2.3

00:00:53

passed #4036
dune:2.4

00:00:36

passed #4037
dune:2.5

00:00:28

passed #4038
dune:git

00:07:58

 
  Stage3
passed #4039
dune-fufem:2.4

00:03:07

passed #4040
dune-fufem:git

00:03:29

 
  Publish
passed #4041
publish

00:40:09