ci/Dockerfile: Pass --no-cache in build example

Using the cache means we don't get updates from Fedora or copr.
5 jobs for !726 with wip/dockerfile-no-cache in 18 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
failed #391560
check-commit-log

00:00:18

 
  Build
skipped #391561
build-mutter
skipped #391562
build-without-native-backend
 
  Test
skipped #391564
can-build-gnome-shell
skipped #391563
test-mutter
 
Name Stage Failure
failed
check-commit-log Review
Checking out a3a0f73f as wip/dockerfile-no-cache...

Skipping Git submodules setup
Authenticating with credentials from job payload (GitLab Registry)
$ ./.gitlab-ci/check-commit-log.sh
From https://gitlab.gnome.org/GNOME/mutter
* branch master -> FETCH_HEAD
Missing merge request or issue URL on commit a3a0f73f
ERROR: Job failed: exit code 1