.gitlab-ci.yml: Use fixed version of Docker image

For some reason `make` has vanished from newer images.

.gitlab-ci.yml: Use fixed version of Docker image
bc6db43e · Sam Thursfield · d1a3c489 · bc6db43e
Commit bc6db43e authored 7 years ago by Sam Thursfield
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
-image: buildstream/buildstream-fedora:latest
+image: buildstream/buildstream-fedora:master-26-3fd7fe1

 cache:
  paths: