feat(Makefile): set docker build flags via environment variable

Author: mboersma

.travis.yml

@@ -7,6 +7,8 @@ branches:
 services:
   - postgresql
   - docker
+env:
+  - DOCKER_BUILD_FLAGS="--pull --no-cache"
 sudo: required
 addons:
   postgresql: "9.4"

versioning.mk

@@ -1,6 +1,5 @@
 MUTABLE_VERSION ?= canary
 VERSION ?= git-$(shell git rev-parse --short HEAD)
-DOCKER_BUILD_FLAGS ?= --no-cache
 
 IMAGE := ${DEIS_REGISTRY}${IMAGE_PREFIX}/${SHORT_NAME}:${VERSION}
 MUTABLE_IMAGE := ${DEIS_REGISTRY}${IMAGE_PREFIX}/${SHORT_NAME}:${MUTABLE_VERSION}