Remove uneeded variables from single.Dockefile
Follow up from !2751 (merged)
Closes #1204 (closed)
To review locally:
export DOCKERFILE=dockerfiles/single.Dockerfile NANOC_ENV=production VER=15.0 CI_COMMIT_REF_NAME=15.0
docker build --build-arg NANOC_ENV=${NANOC_ENV} --build-arg CI_COMMIT_REF_NAME=${CI_COMMIT_REF_NAME} --build-arg VER=${VER} --tag docs:${VER} -f $DOCKERFILE .
Edited by Achilleas Pipinellis