[ci] use plan docker build in CI

.gitlab-ci.yml

@@ -28,6 +28,7 @@ variables:
     variables:
         DOCKER_HOST: tcp://docker:2375/
         DOCKER_DRIVER: overlay2
+        DOCKER_BUILD: build
     before_script:
         - apk --update add py3-pip m4 git file bash python3 curl make tar
         - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY

rules.mk

@@ -10,7 +10,7 @@ THISDIR=$(dir $(lastword $(MAKEFILE_LIST)))
 
 REPONAMES = $(patsubst %/,%,$(dir $(wildcard */Dockerfile.in)))
 DOCKER_NOCACHE=
-DOCKER_BUILD=buildx build --load
+DOCKER_BUILD?=buildx build --load
 BUILD_CMD=$(DOCKER_SUDO) docker $(DOCKER_BUILD) ${DOCKER_NOCACHE} --rm=true ${DOCKER_QUIET}
 REGISTRY=zivgitlab.wwu.io/ag-ohlberger/dune-community/docker
 GITREV?=$(shell git log -1 --pretty=format:"%H")