:wrench: Moved all elements related to docker build in contrib

As requested by maintainer

.travis.yml

@@ -3,4 +3,4 @@ dist: trusty
 services:
   - docker
 script:
-  - make docker_build
+  - make -C contrib docker_build

Makefile

@@ -36,6 +36,3 @@ $(EXAMPLE_TARGETS): examples/build/%: examples/%.cpp matplotlibcpp.h
 
 clean:
 	rm -f ${EXAMPLE_TARGETS}
-
-docker_build:
-	docker build . -t matplotlibcpp

contrib/Makefile

@@ -0,0 +1,6 @@
+all: docker_build
+
+docker_build:
+	cd .. && \
+	docker build . -f contrib/Dockerfile -t matplotlibcpp && \
+	cd contrib