.gitlab-ci.yml: Set correct environment for Docker in Docker to work

Set according to Gitlab CI documentation.
https://docs.gitlab.com/ee/ci/docker/using_docker_build.html

Fixes such error:
https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/jobs/251032471
10 jobs for valentindavid/docker-dind-fix in 169 minutes and 29 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Flatpak
passed #251250201
aarch64
app_aarch64

00:51:42

passed #251250202
armhf
app_arm

00:31:22

passed #251250200
cache_i686 x86_64
app_i686

00:51:31

passed #251250203
ppc64le allowed to fail
app_powerpc64le

00:29:39

passed #251250199
cache_x86_64 x86_64
app_x86_64

02:15:38

 
  Vm
manual #251250206
aarch64 allowed to fail manual
minimal_systemd_vm_aarch64
passed #251250207
armhf
minimal_systemd_vm_arm

00:10:02

passed #251250205
cache_i686 x86_64
minimal_systemd_vm_i686

00:31:42

manual #251250208
ppc64le allowed to fail manual
minimal_systemd_vm_powerpc64le
passed #251250204
cache_x86_64 x86_64
minimal_systemd_vm_x86_64

00:33:49