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.
2 jobs for ci in 50 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #11042
docker
build

00:00:50

failed #11041
docker
build

00:00:06