Makefile: clean git branch before building

Signed-off-by: Antonio Murdaca <runcom@redhat.com>

Makefile

@@ -40,8 +40,9 @@ DOCKER_MOUNT := $(if $(BIND_DIR),-v "$(CURDIR)/$(BIND_DIR):/go/src/github.com/do
 DOCKER_MOUNT := $(if $(DOCKER_MOUNT),$(DOCKER_MOUNT),-v "/go/src/github.com/docker/docker/bundles")
 
 GIT_BRANCH := $(shell git rev-parse --abbrev-ref HEAD 2>/dev/null)
-DOCKER_IMAGE := docker-dev$(if $(GIT_BRANCH),:$(GIT_BRANCH))
-DOCKER_DOCS_IMAGE := docker-docs$(if $(GIT_BRANCH),:$(GIT_BRANCH))
+GIT_BRANCH_CLEAN := $(shell echo $(GIT_BRANCH) | sed -e "s/[^[:alnum:]]/-/g")
+DOCKER_IMAGE := docker-dev$(if $(GIT_BRANCH_CLEAN),:$(GIT_BRANCH_CLEAN))
+DOCKER_DOCS_IMAGE := docker-docs$(if $(GIT_BRANCH_CLEAN),:$(GIT_BRANCH_CLEAN))
 
 DOCKER_FLAGS := docker run --rm -i --privileged $(DOCKER_ENVS) $(DOCKER_MOUNT)