Add a build phase to roll a "pristine" Devuan base image

This runs `debootstrap` in the devuan/migrated image.

.gitignore

@@ -0,0 +1 @@
+/rootfs

.gitlab-ci.yml

@@ -5,16 +5,21 @@
 
 variables:
   DEBIAN_VERSION: jessie-20170723
+  DEVUAN_VERSION: jessie
 
 image: docker:latest
 services:
 - docker:dind
 
+stages:
+  - migrate
+  - build
+
 before_script:
 - docker login -u gitlab-ci-token -p "$CI_JOB_TOKEN" registry.gitlab.com
 
 migrate:
-  stage: build
+  stage: migrate
   script:
   - docker pull "$CI_REGISTRY_IMAGE"/migrated:"$DEBIAN_VERSION" || true
   - docker build --build-arg DEBIAN_VERSION="$DEBIAN_VERSION"
@@ -24,3 +29,17 @@ migrate:
   - docker tag  "$CI_REGISTRY_IMAGE"/migrated:"$DEBIAN_VERSION"
                 "$CI_REGISTRY_IMAGE"/migrated
   - docker push "$CI_REGISTRY_IMAGE"/migrated
+
+build:
+  stage: build
+  script:
+  - docker pull "$CI_REGISTRY_IMAGE"/migrated
+  - rm -rf rootfs || true
+  - docker run --rm --cap-add SYS_ADMIN
+           --volume $PWD:/devuan --workdir /devuan
+           "$CI_REGISTRY_IMAGE"/migrated ./bootstrap.sh
+  - version="$DEVUAN_VERSION"-$(date +%F)
+    && tar -cC rootfs . | docker import - "$(CI_REGISTRY_IMAGE)":"$version"
+    && docker push "$CI_REGISTRY_IMAGE":"$version"
+    && docker tag  "$CI_REGISTRY_IMAGE":"$version" "$CI_REGISTRY_IMAGE"
+    && docker push "$CI_REGISTRY_IMAGE"