Add .PHONY for each makefile instruction

Makes it much easier to keep this maintained properly. See also:

http://clarkgrubb.com/makefile-style-guide#phony-targets

Replaces #1164
Signed-off-by: Ben Firshman <ben@firshman.co.uk>

Makefile

-.PHONY: all build test integration-test unit-test build-py3 unit-test-py3 integration-test-py3
-
+.PHONY: all
 all: test
 
+.PHONY: clean
 clean:
 	-docker rm -vf dpy-dind
 	find -name "__pycache__" | xargs rm -rf
 
+.PHONY: build
 build:
 	docker build -t docker-py .
 
+.PHONY: build-py3
 build-py3:
 	docker build -t docker-py3 -f Dockerfile-py3 .
 
+.PHONY: build-docs
 build-docs:
 	docker build -t docker-py-docs -f Dockerfile-docs .
 
+.PHONY: build-dind-certs
 build-dind-certs:
 	docker build -t dpy-dind-certs -f tests/Dockerfile-dind-certs .
 
+.PHONY: test
 test: flake8 unit-test unit-test-py3 integration-dind integration-dind-ssl
 
+.PHONY: unit-test
 unit-test: build
 	docker run docker-py py.test tests/unit
 
+.PHONY: unit-test-py3
 unit-test-py3: build-py3
 	docker run docker-py3 py.test tests/unit
 
+.PHONY: integration-test
 integration-test: build
 	docker run -v /var/run/docker.sock:/var/run/docker.sock docker-py py.test tests/integration
 
+.PHONY: integration-test-py3
 integration-test-py3: build-py3
 	docker run -v /var/run/docker.sock:/var/run/docker.sock docker-py3 py.test tests/integration
 
+.PHONY: integration-dind
 integration-dind: build build-py3
 	docker rm -vf dpy-dind || :
 	docker run -d --name dpy-dind --privileged dockerswarm/dind:1.12.0 docker daemon\
@@ -42,6 +52,7 @@ integration-dind: build build-py3
 		py.test tests/integration
 	docker rm -vf dpy-dind
 
+.PHONY: integration-dind-ssl
 integration-dind-ssl: build-dind-certs build build-py3
 	docker run -d --name dpy-dind-certs dpy-dind-certs
 	docker run -d --env="DOCKER_HOST=tcp://localhost:2375" --env="DOCKER_TLS_VERIFY=1"\
@@ -57,8 +68,10 @@ integration-dind-ssl: build-dind-certs build build-py3
 		--link=dpy-dind-ssl:docker docker-py3 py.test tests/integration
 	docker rm -vf dpy-dind-ssl dpy-dind-certs
 
+.PHONY: flake8
 flake8: build
 	docker run docker-py flake8 docker tests
 
+.PHONY: docs
 docs: build-docs
 	docker run -v `pwd`/docs:/home/docker-py/docs/ -p 8000:8000 docker-py-docs mkdocs serve -a 0.0.0.0:8000