gitlab-ci: Fix build environment.

Replace the `alpine` tag with an explicit `alpine` container
version. This will dispatch on the currently available ci
runners in the upstream gitlab repository.

We prefer an explicit version so we have more control over
updates to the testing environment. This will need to be
bumped periodically.

.gitlab-ci.yml

@@ -4,9 +4,10 @@ build:
     GIT_SUBMODULE_STRATEGY: recursive
 
   tags:
-    - alpine
     - docker
 
+  image: alpine:3.12
+
   stage: build
 
   before_script: