Disable IPv6 tests of Docker image by default

docker/coturn/Makefile

@@ -176,6 +176,7 @@ docker.push:
 #		[( [build=no]
 #		 | build=yes [DOCKERFILE=(debian|alpine)]
 #		             [ref=<git-ref>] )]
+#		[with=ipv6]
 
 test-docker-platforms = $(strip $(if $(call eq,$(platforms),),$(MAIN_PLATFORM),\
                                 $(if $(call eq,$(platforms),@all),$(PLATFORMS),\
@@ -195,6 +196,7 @@ define test.docker.do
 		@make docker.image DOCKERFILE=$(DOCKERFILE) \
 			no-cache=no tag=$(tag) platform=$(platform) ref=$(ref) ,)
 	IMAGE=coturn/$(NAME):$(tag) PLATFORM=$(platform) \
+	$(if $(call eq,$(with),ipv6),TEST_IPV6=1,) \
 	node_modules/.bin/bats \
 		--timing $(if $(call eq,$(CI),),--pretty,--formatter tap) \
 		tests/main.bats

docker/coturn/tests/main.bats

@@ -149,6 +149,8 @@
 }
 
 @test "detect-external-ip returns valid IPv6" {
+  [ -z "$TEST_IPV6" ] && skip
+
   run docker run --rm --platform $PLATFORM --entrypoint sh $IMAGE -c \
     'detect-external-ip --ipv6'
   [ "$status" -eq 0 ]