Move docs image docker-sphinx to ghcr from docker hub (kanisterio#982)

* Move docs image `docker-sphinx` to ghcr from docker hub

It looks like when we moved kanister images from docker hub to
ghcr, we forgot about `docker-sphinx`.
Our pipeline fails a lot of times because of pull request limits
there and network issues.
Moving that image to ghcr where all of our other images are.

* Re-trigger CI

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Makefile

@@ -58,7 +58,8 @@ IMAGE_NAME := $(BIN)
 IMAGE := $(REGISTRY)/$(IMAGE_NAME)
 
 BUILD_IMAGE ?= ghcr.io/kanisterio/build:v0.0.12
-DOCS_BUILD_IMAGE ?= kanisterio/docker-sphinx
+
+DOCS_BUILD_IMAGE ?= ghcr.io/kanisterio/docker-sphinx
 
 DOCS_RELEASE_BUCKET ?= s3://docs.kanister.io