ci: Use default image tag "latest"

Up until now, our images have been tagged as "master" instead
of the default "latest" due to the way the build process
worked, but we're using the default now.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

.gitlab-ci.yml

@@ -11,36 +11,36 @@
 
 debian-9-cross-armv6l:
   <<: *job_definition
-  image: quay.io/libvirt/buildenv-debian-9-cross-armv6l:master
+  image: quay.io/libvirt/buildenv-debian-9-cross-armv6l:latest
 
 debian-9-cross-mipsel:
   <<: *job_definition
-  image: quay.io/libvirt/buildenv-debian-9-cross-mipsel:master
+  image: quay.io/libvirt/buildenv-debian-9-cross-mipsel:latest
 
 debian-9-cross-ppc64le:
   <<: *job_definition
-  image: quay.io/libvirt/buildenv-debian-9-cross-ppc64le:master
+  image: quay.io/libvirt/buildenv-debian-9-cross-ppc64le:latest
 
 debian-9-cross-s390x:
   <<: *job_definition
-  image: quay.io/libvirt/buildenv-debian-9-cross-s390x:master
+  image: quay.io/libvirt/buildenv-debian-9-cross-s390x:latest
 
 debian-sid-cross-aarch64:
   <<: *job_definition
-  image: quay.io/libvirt/buildenv-debian-sid-cross-aarch64:master
+  image: quay.io/libvirt/buildenv-debian-sid-cross-aarch64:latest
 
 debian-sid-cross-armv7l:
   <<: *job_definition
-  image: quay.io/libvirt/buildenv-debian-sid-cross-armv7l:master
+  image: quay.io/libvirt/buildenv-debian-sid-cross-armv7l:latest
 
 debian-sid-cross-i686:
   <<: *job_definition
-  image: quay.io/libvirt/buildenv-debian-sid-cross-i686:master
+  image: quay.io/libvirt/buildenv-debian-sid-cross-i686:latest
 
 debian-sid-cross-mips64el:
   <<: *job_definition
-  image: quay.io/libvirt/buildenv-debian-sid-cross-mips64el:master
+  image: quay.io/libvirt/buildenv-debian-sid-cross-mips64el:latest
 
 debian-sid-cross-mips:
   <<: *job_definition
-  image: quay.io/libvirt/buildenv-debian-sid-cross-mips:master
+  image: quay.io/libvirt/buildenv-debian-sid-cross-mips:latest

Makefile.ci

@@ -53,7 +53,7 @@ CI_IMAGE_PREFIX = quay.io/libvirt/buildenv-
 
 # The default tag is ':latest' but if the container
 # repo above uses different conventions this can override it
-CI_IMAGE_TAG = :master
+CI_IMAGE_TAG = :latest
 
 # We delete the virtual root after completion, set
 # to 0 if you need to keep it around for debugging