CI: do the rebuilds via kaniko.

1 job for ci-changes in 17 seconds (queued for 38 seconds)
Status Job ID Name Coverage
  Build
failed #241251
build

00:00:17

 
Name Stage Failure
failed
build Build
++ '[' -d dav1d-debian-unstable ']'
Changed images: ' dav1d-debian-unstable'
+++ date +%Y%m%d%H%M%S
++ DATE=20181206211831
++ /kaniko/executor --no-push --context dav1d-debian-unstable --dockerfile Dockerfile --destination registry.videolan.org:5000/dav1d-debian-unstable-aarch64:20181206211831
INFO[0000] Downloading base image armv8a/debian:sid-20181112
2018/12/06 21:18:32 No matching credentials were found, falling back on anonymous
error building image: getting stage builder for stage 0: UNAUTHORIZED: "authentication required"
ERROR: Job failed: exit code 1