similar to the changes made in a4192471cd6d6012a66c19a635a11c7f6139d683,
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>