template: add '--format docker' as default build arg

Without this build arg images build with the build-ah-engine container 0.3.0 and podman won't work on docker hosts 1.13.1 which is the current stable on CentOS 7 LTS.

Adding this format build arg ensures the widest range of registry support, thus this should be the default.

I propose a 0.3.2 maintenance release.

gitlab-ci-template.yml

@@ -7,6 +7,8 @@ variables:
 
 container-build:
   stage: build
+  variables:
+    CI_REGISTRY_CONTAINER_FORMAT: "docker"
   inherit:
     default: false
     variables:
@@ -24,7 +26,7 @@ container-build:
     - export VCS_URL=$CI_PROJECT_URL
   script:
     - si-fix "${CI_REGISTRY_BUILD_DOCKERFILE:-./Dockerfile}"
-    - podman build --pull
+    - podman build --pull --format "$CI_REGISTRY_CONTAINER_FORMAT"
       --label "org.opencontainers.image.source=$CI_PROJECT_URL"
       --label "org.opencontainers.image.revision=$CI_COMMIT_SHA"
       --label "org.opencontainers.image.created=$(date --rfc-3339 ns)"