.gitlab-ci.yml: Switch to our own Docker images

By building our own images with common dependencies already installed,
we can make actual test runs faster and less wasteful. That said, do
keep directives for installing dependencies here in case of some of them
having been updated since the last rebuild of the images.

.gitlab-ci.yml

@@ -7,6 +7,8 @@ stages:
 #
 
 .ensembl_test_template:
+  image: dockerhub.ebi.ac.uk/ensembl-infrastructure/ensembl-ci-docker-images:${PERL_VERSION}
+
   services:
     - mysql:5.6
 
@@ -46,8 +48,8 @@ stages:
 test:perl5.14-sqlite:
   stage: test
   extends: .ensembl_test_template
-  image: perl:5.14
   variables:
+    PERL_VERSION: "5.14"
     COVERALLS: "false"
     DB: "sqlite"
   script:
@@ -56,8 +58,8 @@ test:perl5.14-sqlite:
 test:perl5.30-mysql:
   stage: test
   extends: .ensembl_test_template
-  image: perl:5.30
   variables:
+    PERL_VERSION: "5.30"
     # Note: relies on the secret variable COVERALLS_REPO_TOKEN for report uploads to work
     COVERALLS: "true"
     DB: "mysql"