fix docs not building if branch-name contains slashes

similar to the changes made in a4192471cd,
clean the branch-name before using it as a tag-name
for the image that's built.

this also changes the "image name" for docs images
being built from "docs-base" to "docker-docs", to
be in line with the main Makefile

before this change:

  /usr/bin/make -C docs docs
  docker build -t "docs-base:docs/slashes" .
  invalid value "docs-base:docs/slashes" for flag -t: Error parsing reference: "docs-base:docs/slashes" is not a valid repository/tag
  See 'docker build --help'.
  make[1]: *** [docs-build] Error 125
  make: *** [docs] Error 2

after this change:

  /usr/bin/make -C docs docs
  docker build -t "docker-docs:docs-slashes" .
  Sending build context to Docker daemon 9.165 MB
  ...

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/Makefile

@@ -23,9 +23,8 @@ HUGO_BASE_URL=$(shell test -z "$(DOCKER_IP)" && echo localhost || echo "$(DOCKER
 HUGO_BIND_IP=0.0.0.0
 
 GIT_BRANCH := $(shell git rev-parse --abbrev-ref HEAD 2>/dev/null)
-DOCKER_IMAGE := docker$(if $(GIT_BRANCH),:$(GIT_BRANCH))
-DOCKER_DOCS_IMAGE := docs-base$(if $(GIT_BRANCH),:$(GIT_BRANCH))
-
+GIT_BRANCH_CLEAN := $(shell echo $(GIT_BRANCH) | sed -e "s/[^[:alnum:]]/-/g")
+DOCKER_DOCS_IMAGE := docker-docs$(if $(GIT_BRANCH_CLEAN),:$(GIT_BRANCH_CLEAN))
 
 DOCKER_RUN_DOCS := docker run --rm -it $(DOCS_MOUNT) -e AWS_S3_BUCKET -e NOCACHE