Shorten Docker image definitions by using YAML anchors for runner tags

While we are at it, drop use of the "docker" tag since all BIND CI jobs
are currently run inside Docker containers.

.gitlab-ci.yml

@@ -11,89 +11,63 @@ stages:
   - test
   - push
 
-.centos-centos6-amd64: &centos_centos6_amd64_image
-  image: "$CI_REGISTRY_IMAGE:centos-centos6-amd64"
+.linux-amd64: &linux_amd64
   tags:
     - linux
-    - docker
     - amd64
 
+.linux-i386: &linux_i386
+  tags:
+    - linux
+    - i386
+
+.centos-centos6-amd64: &centos_centos6_amd64_image
+  image: "$CI_REGISTRY_IMAGE:centos-centos6-amd64"
+  <<: *linux_amd64
+
 .centos-centos7-amd64: &centos_centos7_amd64_image
   image: "$CI_REGISTRY_IMAGE:centos-centos7-amd64"
-  tags:
-    - linux
-    - docker
-    - amd64
+  <<: *linux_amd64
 
 .debian-jessie-amd64: &debian_jessie_amd64_image
   image: "$CI_REGISTRY_IMAGE:debian-jessie-amd64"
-  tags:
-    - linux
-    - docker
-    - amd64
+  <<: *linux_amd64
 
 .debian-jessie-i386: &debian_jessie_i386_image
   image: "$CI_REGISTRY_IMAGE:debian-jessie-i386"
-  tags:
-    - linux
-    - docker
-    - i386
+  <<: *linux_i386
 
 .debian-stretch-amd64: &debian_stretch_amd64_image
   image: "$CI_REGISTRY_IMAGE:debian-stretch-amd64"
-  tags:
-    - linux
-    - docker
-    - amd64
+  <<: *linux_amd64
 
 .debian-stretch-i386: &debian_stretch_i386_image
   image: "$CI_REGISTRY_IMAGE:debian-stretch-i386"
-  tags:
-    - linux
-    - docker
-    - i386
+  <<: *linux_i386
 
 .debian-sid-amd64: &debian_sid_amd64_image
   image: "$CI_REGISTRY_IMAGE:debian-sid-amd64"
-  tags:
-    - linux
-    - docker
-    - amd64
+  <<: *linux_amd64
 
 .debian-sid-i386: &debian_sid_i386_image
   image: "$CI_REGISTRY_IMAGE:debian-sid-i386"
-  tags:
-    - linux
-    - docker
-    - i386
+  <<: *linux_i386
 
 .ubuntu-xenial-amd64: &ubuntu_xenial_amd64_image
   image: "$CI_REGISTRY_IMAGE:ubuntu-xenial-amd64"
-  tags:
-    - linux
-    - docker
-    - amd64
+  <<: *linux_amd64
 
 .ubuntu-xenial-i386: &ubuntu_xenial_i386_image
   image: "$CI_REGISTRY_IMAGE:ubuntu-xenial-i386"
-  tags:
-    - linux
-    - docker
-    - i386
+  <<: *linux_i386
 
 .ubuntu-bionic-amd64: &ubuntu_bionic_amd64_image
   image: "$CI_REGISTRY_IMAGE:ubuntu-bionic-amd64"
-  tags:
-    - linux
-    - docker
-    - amd64
+  <<: *linux_amd64
 
 .ubuntu-bionic-i386: &ubuntu_bionic_i386_image
   image: "$CI_REGISTRY_IMAGE:ubuntu-bionic-i386"
-  tags:
-    - linux
-    - docker
-    - i386
+  <<: *linux_i386
 
 .build: &build_job
   stage: build