Create Docker images for the examples

When those are published on Dockerhub, people will be able to start those
examples in fewer steps than the current documentation:

  https://prometheus.io/docs/introduction/getting_started/#starting-up-some-sample-targets

While relying on Docker may not be always wished, giving the possibility to
quickly start a daemon that exposes metrics can be useful in many tutorials.

This commit tries really hard to limit the number of per-example maintenance
work by sharing most of the Makefile to build the Docker images. The tiny top
level Makefiles in each example directory are strictly identical.

Updates: prometheus#347

.gitignore

@@ -7,6 +7,11 @@
 _obj
 _test
 
+# Examples
+/examples/**/Dockerfile
+/examples/random/random
+/examples/simple/simple
+
 # Architecture specific extensions/prefixes
 *.[568vq]
 [568vq].out

examples/Makefile

@@ -0,0 +1,39 @@
+#
+# This Makefile is included by a top-level Makefile in each example directory
+#
+# Invocation from the root of the project:
+# $ make -C examples/simple
+#
+DOCKER_IMAGE_NAME ?= example-golang-$(EXAMPLE)
+DOCKER_IMAGE_TAG ?= $(subst /,-,$(shell git rev-parse --abbrev-ref HEAD))
+
+all: $(EXAMPLE) docker
+
+$(EXAMPLE): main.go Makefile
+ @echo ">> fetching dependencies"
+ @go get -d
+ @echo ">> building $@"
+ @CGO_ENABLED=0 GOOS=linux go build -a -tags netgo -ldflags '-w'
+
+define DockerfileTemplate
+FROM scratch
+LABEL maintainer "The Prometheus Authors <[email protected]>"
+ADD $(EXAMPLE) $(EXAMPLE)
+EXPOSE 8080
+ENTRYPOINT ["/$(EXAMPLE)"]
+endef
+
+export DockerfileTemplate
+Dockerfile: Makefile
+ @echo ">> writing Dockerfile"
+ @echo "$$DockerfileTemplate" >> $@
+
+docker: Dockerfile
+ @echo ">> building docker image"
+ @docker build -t "$(DOCKER_IMAGE_NAME):$(DOCKER_IMAGE_TAG)" .
+
+clean:
+ @rm -f $(EXAMPLE) Dockerfile
+ @docker rmi -f "$(DOCKER_IMAGE_NAME):$(DOCKER_IMAGE_TAG)"
+
+.PHONY: all docker clean

examples/random/Makefile

@@ -0,0 +1,5 @@
+EXAMPLE_DIRECTORY := $(dir $(realpath $(lastword $(MAKEFILE_LIST))))
+name = $(patsubst %/,%,$(dir $1))
+EXAMPLE := $(notdir $(call name,$(EXAMPLE_DIRECTORY)))
+
+include $(EXAMPLE_DIRECTORY)/../Makefile

examples/simple/Makefile

@@ -0,0 +1,5 @@
+EXAMPLE_DIRECTORY := $(dir $(realpath $(lastword $(MAKEFILE_LIST))))
+name = $(patsubst %/,%,$(dir $1))
+EXAMPLE := $(notdir $(call name,$(EXAMPLE_DIRECTORY)))
+
+include $(EXAMPLE_DIRECTORY)/../Makefile