This was missing from the work to add the alpine container.
Fixes: a9afe7768bd ("automation: add alpine linux 3.12 x86 build container")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>
#
BASE="registry.gitlab.com/xen-project/xen"
case "_${CONTAINER}" in
+ _alpine) CONTAINER="${BASE}/alpine:3.12" ;;
_archlinux|_arch) CONTAINER="${BASE}/archlinux:current" ;;
_centos7) CONTAINER="${BASE}/centos:7" ;;
_centos72) CONTAINER="${BASE}/centos:7.2" ;;