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.