Use DOCKER_CMD from the environment (if defined) in the containers'
makefile too, so that, e.g., when doing `export DOCKED_CMD=podman`
podman is used for building the containers too.

Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
---
Cc: Doug Goldstein <cardoe@cardoe.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
---
Changes from v1:
- fix my email address in From:
---
 automation/build/Makefile |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/automation/build/Makefile b/automation/build/Makefile
index 7c7612b1d9..a4b2b85178 100644
--- a/automation/build/Makefile
+++ b/automation/build/Makefile
@@ -2,6 +2,7 @@
 # the base of where these containers will appear
 REGISTRY := registry.gitlab.com/xen-project/xen
 CONTAINERS = $(subst .dockerfile,,$(wildcard */*.dockerfile))
+DOCKER_CMD ?= docker
 
 help:
 	@echo "Builds containers for building Xen based on different distros"
@@ -10,9 +11,9 @@ help:
 	@echo "To push container builds, set the env var PUSH"
 
 %: %.dockerfile ## Builds containers
-	docker build -t $(REGISTRY)/$(@D):$(@F) -f $< $(<D)
+	$(DOCKER_CMD) build -t $(REGISTRY)/$(@D):$(@F) -f $< $(<D)
 	@if [ ! -z $${PUSH+x} ]; then \
-		docker push $(REGISTRY)/$(@D):$(@F); \
+		$(DOCKER_CMD) push $(REGISTRY)/$(@D):$(@F); \
 	fi
 
 .PHONY: all