Use specific version of Docker dind

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

.gitlab-ci.yml

@@ -183,7 +183,7 @@ pages:
 
 .docker: &docker
   stage: docker
-  image: docker:stable
+  image: docker:20.10.12
   variables:
     DOCKER_TLS_CERTDIR: "/certs"
     DOCKER_HOST: tcp://docker:2376
@@ -191,7 +191,7 @@ pages:
     DOCKER_CERT_PATH: "$DOCKER_TLS_CERTDIR/client"
     DOCKER_DRIVER: overlay2
   services:
-    - docker:stable-dind
+    - docker:20.10.12-dind
   cache: {}
   before_script:
     # Install buildx