tests/docker: make docker engine choice entirely configure driven

Since 0b1a649 (tests/docker: use direct RUNC call to build
containers) we ended up with the potential for the remaining docker.py
script calls to deviate from the direct RUNC calls. Fix this by
dropping the use of ENGINE in the makefile and rely entirely on what
we detect at configure time.

We also tweak the RUNC detection so podman users can still run things
from the source tree.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20231009164104.369749-6-alex.bennee@linaro.org>

configure

@@ -1694,7 +1694,6 @@ if test -n "$gdb_bin"; then
 fi
 
 if test "$container" != no; then
-    echo "ENGINE=$container" >> $config_host_mak
     echo "RUNC=$runc" >> $config_host_mak
 fi
 echo "SUBDIRS=$subdirs" >> $config_host_mak

tests/docker/Makefile.include

@@ -16,9 +16,8 @@ DOCKER_DEFAULT_REGISTRY := registry.gitlab.com/qemu-project/qemu
 endif
 DOCKER_REGISTRY := $(if $(REGISTRY),$(REGISTRY),$(DOCKER_DEFAULT_REGISTRY))
 
-RUNC ?= docker
-ENGINE ?= auto
-DOCKER_SCRIPT=$(SRC_PATH)/tests/docker/docker.py --engine $(ENGINE)
+RUNC ?= $(if $(shell command -v docker), docker, podman)
+DOCKER_SCRIPT=$(SRC_PATH)/tests/docker/docker.py --engine $(RUNC)
 
 CUR_TIME := $(shell date +%Y-%m-%d-%H.%M.%S.$$$$)
 DOCKER_SRC_COPY := $(BUILD_DIR)/docker-src.$(CUR_TIME)
@@ -158,7 +157,7 @@ $(foreach i,$(filter-out $(DOCKER_PARTIAL_IMAGES),$(DOCKER_IMAGES)), \
 )
 
 docker:
-	@echo 'Build QEMU and run tests inside Docker or Podman containers'
+	@echo 'Build QEMU and run tests inside $(RUNC) containers'
 	@echo
 	@echo 'Available targets:'
 	@echo
@@ -198,8 +197,6 @@ docker:
 	@echo '    EXECUTABLE=<path>    Include executable in image.'
 	@echo '    EXTRA_FILES="<path> [... <path>]"'
 	@echo '                         Include extra files in image.'
-	@echo '    ENGINE=auto/docker/podman'
-	@echo '                         Specify which container engine to run.'
 	@echo '    REGISTRY=url         Cache builds from registry (default:$(DOCKER_REGISTRY))'
 
 docker-help: docker