Fix base image name to refer to the right registry

.gitlab-ci.yml

@@ -48,7 +48,7 @@ build:
 derive:
   stage: derive
   script:
-  - docker build --build-arg DEVUAN_VERSION="$DEVUAN_VERSION"
+  - docker build --build-arg DEVUAN_IMAGE="$CI_REGISTRY_IMAGE":"$DEVUAN_VERSION"
            --tag "$CI_REGISTRY_IMAGE"/buildd:"$DEVUAN_VERSION"
            --file buildd.df .
   - docker push "$CI_REGISTRY_IMAGE"/buildd:"$DEVUAN_VERSION"

buildd.df

@@ -3,8 +3,8 @@
 #
 # License: GPL-3.0+
 
-ARG   DEVUAN_VERSION
-FROM  devuan:${DEVUAN_VERSION}
+ARG   DEVUAN_IMAGE
+FROM  ${DEVUAN_IMAGE}
 LABEL maintainer="Olaf Meeuwissen <paddy-hack@member.fsf.org>"
 
 RUN apt-get update -q \