Default to ubuntu BASE_IMAGE in Dockerfile, modify make to only add --build-arg to build target if BASE_IMAGE is defined, document the use of BASE_IMAGE in README

Makefile

@@ -1,10 +1,11 @@
-ifndef BASE_IMAGE
-	BASE_IMAGE = ubuntu:18.04
-	NAME ?= phusion/baseimage
-else ifdef NAME
-else
+ifdef BASE_IMAGE
+	BUILD_ARG = --build-arg BASE_IMAGE=$(BASE_IMAGE)
+ifndef NAME
 	NAME = phusion/baseimage-$(subst :,-,${BASE_IMAGE})
 endif
+else
+	NAME ?= phusion/baseimage
+endif
 VERSION ?= 0.11
 
 
@@ -13,7 +14,7 @@ VERSION ?= 0.11
 all: build
 
 build:
-	docker build -t $(NAME):$(VERSION) --build-arg BASE_IMAGE=$(BASE_IMAGE) --rm image
+	docker build -t $(NAME):$(VERSION) $(BUILD_ARG) --rm image
 
 test:
 	env NAME=$(NAME) VERSION=$(VERSION) ./test/runner.sh

README.md

@@ -597,6 +597,19 @@ If you want to call the resulting image something else, pass the NAME variable,
 
     make build NAME=joe/baseimage
 
+You can also change the `ubuntu` base-image to `debian` as these distributions are quite similar.
+
+    make build BASE_IMAGE=debian:stretch
+
+The image will be: `phusion/baseimage-debian-stretch`. Use the `NAME` variable in combination with the `BASE_IMAGE` one to call it `joe/stretch`.
+
+    make build BASE_IMAGE=debian:stretch NAME=joe/stretch
+
+To verify that the various services are started, when the image is run as a container, add `test` to the end of your make invocations, e.g.:
+
+    make build BASE_IMAGE=debian:stretch NAME=joe/stretch test
+
+
 <a name="removing_optional_services"></a>
 ### Removing optional services
 

image/Dockerfile

@@ -1,4 +1,4 @@
-ARG BASE_IMAGE
+ARG BASE_IMAGE=ubuntu:18.04
 FROM $BASE_IMAGE
 MAINTAINER Phusion <info@phusion.nl>